Bump Go SDK to v0.126.0, remove redundant agent tracking (#4799)

## Why

The Go SDK now includes built-in AI agent detection (PR
databricks/databricks-sdk-go#1537). It appends `agent/<name>` to the
User-Agent header when running inside a known agent environment. The CLI
had its own agent-to-UA wiring that did the same thing, which would now
produce duplicate `agent/` segments.

## Changes

**Before:** The CLI detected agents via `libs/agent` and injected
`agent/<name>` into the UA string in `PersistentPreRunE`. The SDK had no
agent detection.

**Now:** The SDK handles agent detection natively. The CLI's redundant
wiring is removed.

Specific changes:
- Bump Go SDK from v0.120.0 to v0.126.0
- Delete `cmd/root/user_agent_agent.go` (the `withAgentInUserAgent`
function)
- Delete `libs/agent/` package (dead code after removal)
- Remove `agent.Detect(ctx)` from `Execute()` in root.go
- Add HTTP-level tests verifying the SDK produces exactly one `agent/`
segment
- Update auth prompt logic: use `cfg.HostType()` instead of
`ErrNotAccountClient`/`ErrNotWorkspaceClient` (the SDK removed host-type
validation from client constructors in favor of host metadata
resolution). Synthesize sentinel errors when prompting is unavailable to
preserve `MustAnyClient` fallthrough behavior.
- Add `.well-known/databricks-config` handler to the default test
server, so acceptance tests with custom profiles don't produce
non-deterministic host metadata warnings
- Update `auth/switch/nominal` test to point profiles at the mock server
instead of fake DNS hostnames
- Add `.well-known/databricks-config` handlers to labs test mock servers
- Regenerated code from `make generate` with updated OpenAPI SHA

## Test plan

- [x] `TestSDKAgentDetection`: exactly one `agent/claude-code` in UA
header
- [x] `TestSDKNoAgentDetected`: no `agent/` when no agent env vars set
- [x] `TestSDKMultipleAgentsSuppressed`: no `agent/` when multiple
agents detected
- [x] `TestAccountClientOrPrompt` and `TestWorkspaceClientOrPrompt` pass
- [x] `TestAccountClientOrPromptReturnsErrorForWrongHostType`: wrong
host type with prompting disabled returns `ErrNotAccountClient`
- [x] `TestWorkspaceClientOrPromptReturnsErrorForWrongHostType`: wrong
host type with prompting disabled returns `ErrNotWorkspaceClient`
- [x] `TestAccountClientOrPromptReturnsErrorForMissingAccountID`:
missing account ID with prompting disabled returns `ErrNotAccountClient`
- [x] `cmd/labs/project` tests pass
- [x] `experimental/aitools` tests pass
- [x] `make checks` passes
- [x] Acceptance tests (refschema, account-help, auth/switch/nominal)
pass
- [x] `make generate` on final commit produces no diff

This pull request was AI-assisted by Isaac.

Author: simonfaltum

.codegen/_openapi_sha

@@ -1 +1 @@
-a7c320a6b531263c8fa45619c1565b63849750e5
+d09dbd77f5a9560cbb816746773da43a8bdbde08

.gitattributes

@@ -90,6 +90,7 @@ cmd/workspace/enable-notebook-table-clipboard/enable-notebook-table-clipboard.go
 cmd/workspace/enable-results-downloading/enable-results-downloading.go linguist-generated=true
 cmd/workspace/enhanced-security-monitoring/enhanced-security-monitoring.go linguist-generated=true
 cmd/workspace/entity-tag-assignments/entity-tag-assignments.go linguist-generated=true
+cmd/workspace/environments/environments.go linguist-generated=true
 cmd/workspace/experiments/experiments.go linguist-generated=true
 cmd/workspace/external-lineage/external-lineage.go linguist-generated=true
 cmd/workspace/external-locations/external-locations.go linguist-generated=true

.github/dependabot.yml

@@ -15,3 +15,7 @@ updates:
       - "/.github/actions/setup-build-environment"
     schedule:
       interval: "monthly"
+    # tagging.yml is generated and maintained externally. Ignore
+    # actions/create-github-app-token since it is only used in tagging.yml.
+    ignore:
+      - dependency-name: "actions/create-github-app-token"

acceptance/apps/deploy/bundle-with-appname/out.requests.txt

@@ -1,3 +1,7 @@
+{
+  "method": "GET",
+  "path": "/.well-known/databricks-config"
+}
 {
   "method": "POST",
   "path": "/api/2.0/apps/test-app/deployments",

acceptance/apps/deploy/no-bundle-with-appname/out.requests.txt

@@ -1,3 +1,7 @@
+{
+  "method": "GET",
+  "path": "/.well-known/databricks-config"
+}
 {
   "method": "POST",
   "path": "/api/2.0/apps/test-app/deployments",

acceptance/auth/bundle_and_profile/output.txt

@@ -13,6 +13,7 @@
 
 === Inside the bundle, profile flag not matching bundle host. Should use profile from the flag and not the bundle.
 >>> errcode [CLI] current-user me -p profile_name
+Warn: Failed to resolve host metadata: (redacted). Falling back to user config.
 Error: Get "https://non.existing.subdomain.databricks.com/api/2.0/preview/scim/v2/Me": (redacted)
 
 Exit code: 1
@@ -72,6 +73,7 @@ Validation OK!
 
 === Bundle commands load bundle configuration with -t and -p flag, validation not OK (profile host don't match bundle host)
 >>> errcode [CLI] bundle validate -t prod -p DEFAULT
+Warn: Failed to resolve host metadata: (redacted). Falling back to user config.
 Error: cannot resolve bundle auth configuration: the host in the profile ([DATABRICKS_TARGET]) doesn’t match the host configured in the bundle (https://bar.com)
 
 Name: test-auth

acceptance/auth/bundle_and_profile/test.toml

@@ -9,6 +9,10 @@ New='DATABRICKS_TARGET'
 Old='DATABRICKS_URL'
 New='DATABRICKS_TARGET'
 
+[[Repls]]
+Old='Warn: Failed to resolve host metadata: .*\. Falling back to user config\.'
+New='Warn: Failed to resolve host metadata: (redacted). Falling back to user config.'
+
 [[Repls]]
 Old='Get "https://non.existing.subdomain.databricks.com/api/2.0/preview/scim/v2/Me": .*'
 New='Get "https://non.existing.subdomain.databricks.com/api/2.0/preview/scim/v2/Me": (redacted)'

acceptance/auth/credentials/basic/out.requests.txt

@@ -1,3 +1,12 @@
+{
+  "headers": {
+    "User-Agent": [
+      "cli/[DEV_VERSION] databricks-sdk-go/[SDK_VERSION] go/[GO_VERSION] os/[OS]"
+    ]
+  },
+  "method": "GET",
+  "path": "/.well-known/databricks-config"
+}
 {
   "headers": {
     "Authorization": [

acceptance/auth/credentials/oauth/out.requests.txt

@@ -1,3 +1,12 @@
+{
+  "headers": {
+    "User-Agent": [
+      "cli/[DEV_VERSION] databricks-sdk-go/[SDK_VERSION] go/[GO_VERSION] os/[OS]"
+    ]
+  },
+  "method": "GET",
+  "path": "/.well-known/databricks-config"
+}
 {
   "headers": {
     "User-Agent": [

acceptance/auth/credentials/pat/out.requests.txt

@@ -1,3 +1,12 @@
+{
+  "headers": {
+    "User-Agent": [
+      "cli/[DEV_VERSION] databricks-sdk-go/[SDK_VERSION] go/[GO_VERSION] os/[OS]"
+    ]
+  },
+  "method": "GET",
+  "path": "/.well-known/databricks-config"
+}
 {
   "headers": {
     "Authorization": [