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/apidoc/docs.go

@@ -13239,6 +13239,12 @@ const docTemplate = `{
                         "$ref": "#/definitions/codersdk.AIBridgeAgenticAction"
                     }
                 },
+                "credential_hint": {
+                    "type": "string"
+                },
+                "credential_kind": {
+                    "type": "string"
+                },
                 "ended_at": {
                     "type": "string",
                     "format": "date-time"

coderd/apidoc/swagger.json

@@ -11809,6 +11809,12 @@
 						"$ref": "#/definitions/codersdk.AIBridgeAgenticAction"
 					}
 				},
+				"credential_hint": {
+					"type": "string"
+				},
+				"credential_kind": {
+					"type": "string"
+				},
 				"ended_at": {
 					"type": "string",
 					"format": "date-time"