Add method to fetch Host Metadata (#678)

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---------
## Summary

Adds `HostMetadata` and a package-private `getHostMetadata()` on
`DatabricksConfig` for parsing the `/.well-known/databricks-config`
discovery endpoint.

## Why

Databricks hosts expose a standard `/.well-known/databricks-config`
endpoint that returns the OIDC endpoint, account ID, and workspace ID in
a single request. The SDK had no primitive to consume it — OIDC endpoint
discovery was handled entirely through host-type-specific logic that
requires the caller to already know whether the host is a workspace,
account console, or unified host.

This PR introduces the foundational building block: a `HostMetadata`
class and a package-private `getHostMetadata()` on `DatabricksConfig`
that fetches and parses the endpoint. The method returns raw metadata
with no substitution (e.g. `{account_id}` placeholders are left as-is),
keeping it a pure discovery primitive. Callers decide how to interpret
the result.

## What changed

### Interface changes

- **`HostMetadata`** — New class in `com.databricks.sdk.core.oauth` with
fields `oidcEndpoint`, `accountId`, `workspaceId`. Deserialized from
JSON via Jackson.
- **`DatabricksConfig.getHostMetadata()` (package-private)** — Fetches
`{host}/.well-known/databricks-config` and returns a `HostMetadata`.
Throws `DatabricksException` on any HTTP error.

### Behavioral changes

None. No existing code paths are modified.

### Internal changes

Tests in `DatabricksConfigTest` covering the two response shapes
(workspace with static OIDC endpoint, account host with `{account_id}`
template) and the HTTP error path.

## How is this tested?

Unit tests in `DatabricksConfigTest` using `FixtureServer`. Both
workspace and account host response shapes are exercised, plus an HTTP
error case.

NO_CHANGELOG=true

Author: hectorcast-db

databricks-sdk-java/src/main/java/com/databricks/sdk/core/DatabricksConfig.java

@@ -5,6 +5,7 @@
 import com.databricks.sdk.core.http.Request;
 import com.databricks.sdk.core.http.Response;
 import com.databricks.sdk.core.oauth.ErrorTokenSource;
+import com.databricks.sdk.core.oauth.HostMetadata;
 import com.databricks.sdk.core.oauth.OAuthHeaderFactory;
 import com.databricks.sdk.core.oauth.OpenIDConnectEndpoints;
 import com.databricks.sdk.core.oauth.TokenSource;
@@ -800,6 +801,32 @@ public OpenIDConnectEndpoints getDatabricksOidcEndpoints() throws IOException {
     return fetchOidcEndpointsFromDiscovery();
   }
 
+  /**
+   * [Experimental] Fetch the raw Databricks well-known configuration from
+   * {host}/.well-known/databricks-config.
+   *
+   * <p><b>Note:</b> This API is experimental and may change or be removed in future releases
+   * without notice.
+   *
+   * @return Parsed {@link HostMetadata} as returned by the server.
+   * @throws DatabricksException if the request fails or the server returns a non-200 status.
+   */
+  HostMetadata getHostMetadata() throws IOException {
+    String url = host + "/.well-known/databricks-config";
+    try {
+      Request request = new Request("GET", url);
+      Response resp = getHttpClient().execute(request);
+      if (resp.getStatusCode() != 200) {
+        throw new DatabricksException(
+            "Failed to fetch host metadata from " + url + ": HTTP " + resp.getStatusCode());
+      }
+      return new ObjectMapper().readValue(resp.getBody(), HostMetadata.class);
+    } catch (IOException e) {
+      throw new DatabricksException(
+          "Failed to fetch host metadata from " + url + ": " + e.getMessage(), e);
+    }
+  }
+
   private OpenIDConnectEndpoints fetchOidcEndpointsFromDiscovery() {
     try {
       Request request = new Request("GET", discoveryUrl);

databricks-sdk-java/src/main/java/com/databricks/sdk/core/oauth/HostMetadata.java

@@ -0,0 +1,42 @@
+package com.databricks.sdk.core.oauth;
+
+import com.fasterxml.jackson.annotation.JsonIgnoreProperties;
+import com.fasterxml.jackson.annotation.JsonProperty;
+
+/**
+ * [Experimental] Parsed response from the /.well-known/databricks-config discovery endpoint.
+ *
+ * <p><b>Note:</b> This API is experimental and may change or be removed in future releases without
+ * notice.
+ */
+@JsonIgnoreProperties(ignoreUnknown = true)
+public class HostMetadata {
+  @JsonProperty("oidc_endpoint")
+  private String oidcEndpoint;
+
+  @JsonProperty("account_id")
+  private String accountId;
+
+  @JsonProperty("workspace_id")
+  private String workspaceId;
+
+  public HostMetadata() {}
+
+  public HostMetadata(String oidcEndpoint, String accountId, String workspaceId) {
+    this.oidcEndpoint = oidcEndpoint;
+    this.accountId = accountId;
+    this.workspaceId = workspaceId;
+  }
+
+  public String getOidcEndpoint() {
+    return oidcEndpoint;
+  }
+
+  public String getAccountId() {
+    return accountId;
+  }
+
+  public String getWorkspaceId() {
+    return workspaceId;
+  }
+}

databricks-sdk-java/src/test/java/com/databricks/sdk/core/DatabricksConfigTest.java

@@ -6,6 +6,7 @@
 import com.databricks.sdk.core.commons.CommonsHttpClient;
 import com.databricks.sdk.core.http.HttpClient;
 import com.databricks.sdk.core.oauth.ErrorTokenSource;
+import com.databricks.sdk.core.oauth.HostMetadata;
 import com.databricks.sdk.core.oauth.OAuthHeaderFactory;
 import com.databricks.sdk.core.oauth.OpenIDConnectEndpoints;
 import com.databricks.sdk.core.oauth.Token;
@@ -421,4 +422,61 @@ public void testGetClientTypeAccountOnUnified() {
             .setExperimentalIsUnifiedHost(true)
             .getClientType());
   }
+
+  // --- HostMetadata tests ---
+
+  private static final String DUMMY_ACCOUNT_ID = "00000000-0000-0000-0000-000000000001";
+  private static final String DUMMY_WORKSPACE_ID = "111111111111111";
+
+  private static Environment emptyEnv() {
+    return new Environment(new HashMap<>(), new ArrayList<>(), System.getProperty("os.name"));
+  }
+
+  @Test
+  public void testGetHostMetadataWorkspaceStaticOidcEndpoint() throws IOException {
+    String response =
+        "{\"oidc_endpoint\":\"https://ws.databricks.com/oidc\","
+            + "\"account_id\":\""
+            + DUMMY_ACCOUNT_ID
+            + "\","
+            + "\"workspace_id\":\""
+            + DUMMY_WORKSPACE_ID
+            + "\"}";
+    try (FixtureServer server =
+        new FixtureServer().with("GET", "/.well-known/databricks-config", response, 200)) {
+      DatabricksConfig config = new DatabricksConfig().setHost(server.getUrl());
+      config.resolve(emptyEnv());
+      HostMetadata meta = config.getHostMetadata();
+      assertEquals("https://ws.databricks.com/oidc", meta.getOidcEndpoint());
+      assertEquals(DUMMY_ACCOUNT_ID, meta.getAccountId());
+      assertEquals(DUMMY_WORKSPACE_ID, meta.getWorkspaceId());
+    }
+  }
+
+  @Test
+  public void testGetHostMetadataAccountRawOidcTemplate() throws IOException {
+    String response =
+        "{\"oidc_endpoint\":\"https://acc.databricks.com/oidc/accounts/{account_id}\"}";
+    try (FixtureServer server =
+        new FixtureServer().with("GET", "/.well-known/databricks-config", response, 200)) {
+      DatabricksConfig config = new DatabricksConfig().setHost(server.getUrl());
+      config.resolve(emptyEnv());
+      HostMetadata meta = config.getHostMetadata();
+      assertEquals("https://acc.databricks.com/oidc/accounts/{account_id}", meta.getOidcEndpoint());
+      assertNull(meta.getAccountId());
+      assertNull(meta.getWorkspaceId());
+    }
+  }
+
+  @Test
+  public void testGetHostMetadataRaisesOnHttpError() throws IOException {
+    try (FixtureServer server =
+        new FixtureServer().with("GET", "/.well-known/databricks-config", "{}", 404)) {
+      DatabricksConfig config = new DatabricksConfig().setHost(server.getUrl());
+      config.resolve(emptyEnv());
+      DatabricksException ex =
+          assertThrows(DatabricksException.class, () -> config.getHostMetadata());
+      assertTrue(ex.getMessage().contains("Failed to fetch host metadata"));
+    }
+  }
 }