feat: per-server custom OAuth authorization-request parameters (#2018)

Real deployments gate IdP selection, prompt behavior, or audience on a query
parameter the OAuth/OIDC core specs never standardized (Keycloak's
`kc_idp_hint`, OIDC's `login_hint` / `prompt` / `acr_values`, Auth0's
`audience`), and none of them could be driven from the Inspector.

The Inspector never builds the authorize URL — the SDK's `auth()` does, and it
exposes no hook for extra parameters — so the merge happens at the one seam
that sees the finished URL: `BaseOAuthClientProvider.redirectToAuthorization`.
It lives in `core/`, so web, CLI, and TUI all inherit it.

- `core/auth/authorizationParams.ts`: the reserved-key list (`client_id`,
  `code_challenge`, `code_challenge_method`, `redirect_uri`, `resource`,
  `response_type`, `scope`, `state`), the key validator the form renders
  inline, and the merge itself. Reserved keys are dropped with a warning
  rather than silently overriding PKCE / the state binding / RFC 8707;
  blank-key rows are skipped; the caller's URL is never mutated.
- Applied to the authorization request only — nothing wires it into the token
  request.
- Persisted as `oauth.authorizationParams` (a string record) on the per-server
  config beside `scopes`, edited as key/value rows in Server Settings ->
  Authorization ("Additional authorization parameters"), with an inline error
  per offending row plus a section-level warning.

- The enterprise-managed (EMA) leg redirects to a *different* authorization
  server (the enterprise IdP), so it goes through the explicit
  `redirectToExternalAuthorization` entry point, which skips the merge — a
  `kc_idp_hint` or `audience` meant for the MCP server's AS must never be
  appended to the IdP's authorize URL.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RDXCSrnSBV7vZfxycG5Z6E
Signed-off-by: cliffhall <cliff@futurescale.com>

core/mcp/oauthManager.ts

@@ -94,6 +94,7 @@ export class OAuthManager {
     clientMetadataUrl?: string;
     scope?: string;
     enterpriseManaged?: boolean;
+    authorizationParams?: Record<string, string>;
   }): void {
     this.oauthConfig = {
       ...this.oauthConfig,
@@ -135,6 +136,11 @@ export class OAuthManager {
       redirectUrlProvider: this.oauthConfig.redirectUrlProvider,
       navigation: this.oauthConfig.navigation,
       clientMetadataUrl: this.oauthConfig.clientMetadataUrl,
+      // #2018: per-server custom authorization-request parameters. Carried on
+      // the provider (not in OAuth storage like `scope`) — they are pure config
+      // read from mcp.json, and nothing in the flow needs to persist or union
+      // them.
+      authorizationParams: this.oauthConfig.authorizationParams,
     });
 
     provider.setEventTarget(this.params.getEventTarget());