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>

clients/web/src/test/integration/mcp/remote/server-extra-coverage.test.ts

@@ -701,6 +701,58 @@ describe("server.ts supplemental coverage", () => {
         await stop(h);
       }
     });
+
+    // #2018 — authorizationParams must be a string record; a hand-edited file
+    // with anything else drops the whole `oauth` node rather than feeding
+    // non-string values to the authorize-URL merge.
+    it("drops oauth whose authorizationParams is not a string record", async () => {
+      const h = await start({
+        seedConfig: JSON.stringify({
+          mcpServers: {
+            srv: {
+              type: "streamable-http",
+              url: "https://x.test/mcp",
+              oauth: { authorizationParams: { kc_idp_hint: 5 } },
+            },
+          },
+        }),
+      });
+      try {
+        const res = await fetch(`${h.baseUrl}/api/servers`);
+        expect(res.status).toBe(200);
+        const body = (await res.json()) as {
+          mcpServers: Record<string, Record<string, unknown>>;
+        };
+        expect(body.mcpServers.srv).not.toHaveProperty("oauth");
+      } finally {
+        await stop(h);
+      }
+    });
+
+    it("keeps a well-formed authorizationParams record on read", async () => {
+      const h = await start({
+        seedConfig: JSON.stringify({
+          mcpServers: {
+            srv: {
+              type: "streamable-http",
+              url: "https://x.test/mcp",
+              oauth: { authorizationParams: { kc_idp_hint: "corp" } },
+            },
+          },
+        }),
+      });
+      try {
+        const res = await fetch(`${h.baseUrl}/api/servers`);
+        const body = (await res.json()) as {
+          mcpServers: Record<string, Record<string, unknown>>;
+        };
+        expect(body.mcpServers.srv?.oauth).toEqual({
+          authorizationParams: { kc_idp_hint: "corp" },
+        });
+      } finally {
+        await stop(h);
+      }
+    });
   });
 
   describe("plaintext-secret migration over a mixed config", () => {

clients/web/src/test/integration/mcp/remote/servers-route.test.ts

@@ -449,6 +449,54 @@ describe("/api/servers routes", () => {
       });
     });
 
+    // #2018 — the authorization-parameter rows travel as a pair array on the
+    // wire and land as a record under `oauth` on disk, mirroring `headers`.
+    it("persists custom authorization params under oauth on POST", async () => {
+      const res = await fetch(`${h.baseUrl}/api/servers`, {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({
+          id: "authparams",
+          config: { type: "streamable-http", url: "https://x.test/mcp" },
+          settings: {
+            headers: [],
+            metadata: [],
+            connectionTimeout: 0,
+            requestTimeout: 0,
+            oauthAuthorizationParams: [
+              { key: "kc_idp_hint", value: "corp" },
+              { key: "", value: "dropped" },
+            ],
+          },
+        }),
+      });
+      expect(res.status).toBe(200);
+      const stored = readConfig(h.configPath).mcpServers
+        .authparams as unknown as Record<string, unknown>;
+      expect(stored.oauth).toEqual({
+        authorizationParams: { kc_idp_hint: "corp" },
+      });
+    });
+
+    it("rejects a malformed oauthAuthorizationParams payload", async () => {
+      const res = await fetch(`${h.baseUrl}/api/servers`, {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({
+          id: "badauthparams",
+          config: { type: "streamable-http", url: "https://x.test/mcp" },
+          settings: {
+            headers: [],
+            metadata: [],
+            connectionTimeout: 0,
+            requestTimeout: 0,
+            oauthAuthorizationParams: { kc_idp_hint: "corp" },
+          },
+        }),
+      });
+      expect(res.status).toBe(400);
+    });
+
     it("updates Inspector-extension fields at the top level on PUT", async () => {
       writeFileSync(
         h.configPath,