Add OIDC endpoint helper tests (#679)

## 🥞 Stacked PR
Use this
[link](https://github.com/databricks/databricks-sdk-java/pull/679/files)
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-
[**stack/config-auto-complete-2**](#679)
[[Files
changed](https://github.com/databricks/databricks-sdk-java/pull/679/files)]
-
[stack/config-auto-complete-3](#680)
[[Files
changed](https://github.com/databricks/databricks-sdk-java/pull/680/files/3b4e6d886efaf9fdedf7de2a5d356ea7eea18617..733d3aefce14ba9aac8ecc82e694f9c2e85fda3e)]

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

Adds tests covering OIDC endpoint resolution directly from a discovery
URL, exercising the existing `discoveryUrl` config field and the
`getDatabricksOidcEndpoints()` short-circuit path.

## Why

The OIDC discovery URL returned by `/.well-known/databricks-config`
points to a standard OAuth 2.0 authorization server metadata document
(`authorization_endpoint`, `token_endpoint`). The Java SDK already has
`discoveryUrl` (`DATABRICKS_DISCOVERY_URL`) and
`getDatabricksOidcEndpoints()` fetching directly from it when set — this
is the Java equivalent of the Python SDK's `get_endpoints_from_url()`
addition. This PR adds the missing test coverage for those paths.

## What changed

### Interface changes

None. The `discoveryUrl` config field (`DATABRICKS_DISCOVERY_URL`) and
the `getDatabricksOidcEndpoints()` short-circuit were already present in
the Java SDK.

### Behavioral changes

None. No existing code paths are modified.

### Internal changes

- `testDiscoveryUrlFromEnv` — verifies `DATABRICKS_DISCOVERY_URL` env
var is loaded into `discoveryUrl`.
- `testDatabricksOidcEndpointsUsesDiscoveryUrl` — verifies
`getDatabricksOidcEndpoints()` short-circuits to fetch directly from
`discoveryUrl` when set.

## How is this tested?

Unit tests in `DatabricksConfigTest` using `FixtureServer`.

NO_CHANGELOG=true

Author: hectorcast-db

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

@@ -479,4 +479,36 @@ public void testGetHostMetadataRaisesOnHttpError() throws IOException {
       assertTrue(ex.getMessage().contains("Failed to fetch host metadata"));
     }
   }
+
+  // --- discoveryUrl / OIDC endpoint tests ---
+
+  @Test
+  public void testDiscoveryUrlFromEnv() {
+    Map<String, String> env = new HashMap<>();
+    env.put("DATABRICKS_DISCOVERY_URL", "https://custom.idp.example.com/oidc");
+    DatabricksConfig config = new DatabricksConfig();
+    config.resolve(new Environment(env, new ArrayList<>(), System.getProperty("os.name")));
+    assertEquals("https://custom.idp.example.com/oidc", config.getDiscoveryUrl());
+  }
+
+  @Test
+  public void testDatabricksOidcEndpointsUsesDiscoveryUrl() throws IOException {
+    String discoveryUrlSuffix = "/oidc";
+    String discoveryUrlResponse =
+        "{\"authorization_endpoint\":\"https://ws.databricks.com/oidc/v1/authorize\","
+            + "\"token_endpoint\":\"https://ws.databricks.com/oidc/v1/token\"}";
+    try (FixtureServer server =
+        new FixtureServer().with("GET", discoveryUrlSuffix, discoveryUrlResponse, 200)) {
+      String discoveryUrl = server.getUrl() + discoveryUrlSuffix;
+      OpenIDConnectEndpoints endpoints =
+          new DatabricksConfig()
+              .setHost(server.getUrl())
+              .setDiscoveryUrl(discoveryUrl)
+              .setHttpClient(new CommonsHttpClient.Builder().withTimeoutSeconds(30).build())
+              .getDatabricksOidcEndpoints();
+      assertEquals(
+          "https://ws.databricks.com/oidc/v1/authorize", endpoints.getAuthorizationEndpoint());
+      assertEquals("https://ws.databricks.com/oidc/v1/token", endpoints.getTokenEndpoint());
+    }
+  }
 }