Adds a new enterprise-only `GET /api/v2/ai-gateway/serve` endpoint that standalone AI Gateway replicas use to connect to `coderd` over a DRPC-over-WebSocket transport, mirroring the existing in-memory path used by the embedded AI Bridge daemon.
- The endpoint upgrades the HTTP connection to a WebSocket, multiplexes it with yamux, and finally serves the three DRPC services (Recorder, MCPConfigurator, Authorizer).
- The `X-AI-Governance-Gateway-Key` header is used for authentication.
- The key is looked up by its hashed secret
- Missing or revoked keys return `401`.
- API version negotiation is enforced via a new `aibridged/proto` version (`v1.0`).
- Incompatible versions return `400`.
- `FeatureAIBridge` entitlement is required.
- Key liveness (`last_used_at`) is recorded immediately on connection and refreshed every 60 seconds while the session remains open.
- When key liveness detects the key was deleted (no rows where updated) session is closed.
#### Small refactors
* The three DRPC service registrations are extracted into `aibridgedserver.Register`, shared by both the in-memory and WebSocket paths.
* The literal `256 * 1024` used as the yamux-aligned WebSocket read limit is replaced with the named constant `drpcsdk.YamuxDefaultStreamWindowSize` in all call sites.
* as noted in review comment https://github.com/coder/coder/pull/26506#discussion_r3461905223 order of `SetReadLimit` and `WebsocketNetConn` calls was fixed.
Paweł Banaszewski