## Why
Synchronous auth header construction could silently return no credentials when managed authentication became unavailable. Callers need an error to avoid sending requests with missing or partial authentication.
## What changed
- Make `AuthProvider::add_auth_headers`, `to_auth_headers`, and auth header telemetry fallible, and propagate failures through request and WebSocket setup.
- Return a transient error when managed authentication is unavailable, and preserve network policy denials through auth error conversion.
- Retry transient MCP authentication errors while treating build errors and policy denials as terminal.
- Cancel local MCP event streams before resolving authentication so cleanup still works when credentials become unavailable.
## Testing
Add regression coverage for rejected partial authentication, async header resolution, terminal auth errors without sending requests, managed account changes, MCP retry classification, and local stream cancellation after authentication becomes unavailable.
GitOrigin-RevId: 39d981e232cba1e302de502a2fa6239101114ed1
## Why
Clients with a gateway sign-in UI need to control when browser authorization starts and inspect credential readiness before making authenticated requests.
## What changed
- Add `initialize.capabilities.explicitGatewayOauth` to require explicit login while allowing existing credentials to refresh. Preserve automatic authorization for clients that omit the capability, and prevent later connections from undoing explicit opt-in.
- Add `account/gatewayOAuth/read`, `account/gatewayOAuth/login`, and `account/gatewayOAuth/cancel`, plus `account/gatewayOAuth/changed` notifications. Send the authorization URL only to the initiating connection and cancel its login on disconnect.
- Check gateway authentication before returning cached results from `model/list`, with restart guidance when provider settings have changed.
- Update protocol schemas, TypeScript and Python bindings, and document capability probing and the sign-in lifecycle.
## Testing
Add tests for passive readiness reads, credential reuse after login, token exchange failures, cancellation and immediate retry, connection ownership, legacy versus explicit login, and model-list authentication checks.
GitOrigin-RevId: a26e116a62c9f3db84dc238a40b7f4e816eb1594
cooper-oai