## Why
Enterprise MCP token exchange and sign-in failures can collapse into generic errors, hiding useful diagnostics such as an OAuth `invalid_scope` rejection or a cancelled sign-in.
## What changed
Introduce `EmaErrorDetail` to carry static operation messages and sanitized SDK diagnostics through token-source errors and enterprise login completion. Exchange failures can report the stage, HTTP status, and allowlisted OAuth error code, such as `IdentityProvider: HTTP 400: invalid_scope`, without retaining arbitrary provider text or raw error chains.
Preserve specific discovery, credential storage, cancellation, and account/configuration mismatch errors instead of replacing them with generic failure messages.
## Testing
Extend tests to cover `invalid_scope` diagnostics, specific sign-in completion errors, and exclusion of credentials from diagnostics even when an outer error context contains sensitive text.
GitOrigin-RevId: e9280a5db3565ea91da0810bab528da5ee088d64
jif
67fb9da4b6