feat: byok observability api (#24207)

## Summary
Exposes `credential_kind` and `credential_hint` on AI Bridge session
threads, making credential metadata visible in the session detail API.
   
Each thread in the `/api/v2/aibridge/sessions/{session_id}` response now
includes:
- `credential_kind`: `centralized` or `byok`
- `credential_hint`: masked credential (e.g. `sk-a...pgAA`)
Values are taken from the thread's root interception.
## Changes

- `codersdk/aibridge.go`: Added `CredentialKind` and `CredentialHint`
fields to `AIBridgeThread`
- `coderd/database/db2sdk/db2sdk.go`: Populated from root interception
in `buildAIBridgeThread`
  - `SessionTimeline.stories.tsx`: Added fields to mock thread data

coderd/database/db2sdk/db2sdk.go

@@ -1240,6 +1240,8 @@ func buildAIBridgeThread(
 	if rootIntc != nil {
 		thread.Model = rootIntc.Model
 		thread.Provider = rootIntc.Provider
+		thread.CredentialKind = string(rootIntc.CredentialKind)
+		thread.CredentialHint = rootIntc.CredentialHint
 		// Get first user prompt from root interception.
 		// A thread can only have one prompt, by definition, since we currently
 		// only store the last prompt observed in an interception.