feat: revoke MCP server OAuth grants at the provider on disconnect (#27300)

Closes
[CODAGT-805](https://linear.app/codercom/issue/CODAGT-805/revoke-oauth-grants-at-the-source-for-mcp-servers).

The experimental MCP server OAuth2 disconnect endpoint previously
deleted only the stored token row, leaving the grant active at the OAuth
provider. This PR adds provider-side token revocation while keeping
local disconnect independent of provider availability.

## Changes

- Add `mcp_server_configs.oauth2_revocation_url` in migration `000547`.
The value can be configured manually, discovered from RFC 8414 metadata,
and managed through the MCP server settings UI. Non-admin responses
redact it with the other OAuth2 fields.
- Revoke the refresh token first through the RFC 7009 endpoint, then
fall back to the access token only for `unsupported_token_type`. Public
clients send `client_id`; confidential clients use
`client_secret_basic`.
- Delete the local token transactionally before best-effort provider
revocation. Callers without a token receive the same response for hidden
and nonexistent config IDs, and provider failures return a generic
warning without exposing provider response bodies.
- Require HTTPS revocation endpoints except for HTTP loopback URLs.
Redirects must preserve the POST and remain on the configured origin.
Redirect errors omit provider-controlled paths and query strings so
reflected token material cannot enter logs.
- Treat `200 OK` and `204 No Content` as completed revocations. `202
Accepted` remains a failure because it does not confirm completion.
- Prevent an in-flight refresh from recreating a token deleted by
disconnect. Refresh persistence now uses an optimistic update keyed by
token ID and `updated_at`; only the OAuth callback can create a token
row. Refresh conflicts reload the current row or clear in-memory auth
when disconnect deleted it.
- Return `{token_revoked, token_revocation_error}` from disconnect,
while retaining SDK compatibility with the legacy `204` response. The UI
surfaces provider revocation failures as warning toasts.
- Document revocation endpoint discovery, HTTPS requirements, and
best-effort disconnect behavior.

No token or no configured revocation URL returns `token_revoked: false`
without an error, so disconnect remains idempotent.

> Updated by Mux, an AI coding agent, on Mike's behalf.

coderd/database/dbgen/dbgen.go

@@ -361,6 +361,7 @@ func MCPServerConfig(t testing.TB, db database.Store, seed database.MCPServerCon
 		OAuth2ClientSecretKeyID: seed.OAuth2ClientSecretKeyID,
 		OAuth2AuthURL:           seed.OAuth2AuthURL,
 		OAuth2TokenURL:          seed.OAuth2TokenURL,
+		OAuth2RevocationURL:     seed.OAuth2RevocationURL,
 		OAuth2Scopes:            seed.OAuth2Scopes,
 		APIKeyHeader:            seed.APIKeyHeader,
 		APIKeyValue:             seed.APIKeyValue,