fix(mcp-oauth): allowlist Cursor's OAuth callbacks so its dynamic registration succeeds (#2225)

* fix(mcp-oauth): allowlist Cursor's OAuth callbacks so its dynamic registration succeeds

Cursor (IDE, CLI, and Grok Bot on top of it) registers three redirect URIs
in one /register request: cursor://anysphere.cursor-mcp/oauth/callback,
https://www.cursor.com/agents/mcp/oauth/callback and
http://localhost:8787/callback. Only the loopback matched a built-in
pattern and /register fails the whole set on any unknown URI, so every
Cursor connection to the URL we hand out in Settings died with
"Redirect URI not allowed". Users cannot self-register the cursor://
form either (the settings panel requires https).

Add a built-in `cursor` provider with the two non-loopback callbacks as
exact matches (no cursor.com prefix), name it "Cursor (Anysphere)" on
the consent page, list the pre-approved clients in the OAuth clients
settings text (sv + en) and the mcp-server rules, and cover the
register, allowlist and consent paths with tests.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DBFeTvQXgMCNXR6fG9drff

* fix(mcp-oauth): show the cursor:// deeplink unverified and let CSP pass its post-consent redirect

Review findings on #2225, one pass:

- Skeptic (correctness), REFUTED: new URL('cursor://...').origin is the
  string "null", so the consent page emitted form-action 'self' null and
  Chromium would block the 303 to the deeplink after Allow. The header
  now uses the scheme-source (cursor:) when the origin is opaque; a test
  pins the header on the cursor:// URI.
- Skeptic (security), CodeRabbit (Major) and Superagent (P2): a custom
  scheme can be claimed by any local app (RFC 8252 section 8.4), so it
  must not be presented as a vendor-verified callback. The deeplink is
  its own provider, cursor_deeplink, rendered "Cursor (Anysphere)" with
  the localhost tag "Din egen dator" and verified: false. The https
  cursor.com callback keeps the verified label. A test pins that a code
  minted without a code_challenge can never be exchanged, which is what
  keeps a scheme hijack from turning into a token.
- CodeRabbit (Minor): the rules doc now says the Grok callback matches
  with or without the trailing slash.
- Regression skeptic: docs/WHITELABEL.md listed only Claude and
  localhost and pointed at the wrong file; now lists the built-ins and
  points at lib/auth/oauth-allowlist.ts.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DBFeTvQXgMCNXR6fG9drff

---------

Co-authored-by: Claude Fable 5.1 <noreply@anthropic.com>

app/api/mcp-oauth/register/__tests__/route.test.ts

@@ -63,6 +63,35 @@ describe('POST /api/mcp-oauth/register', () => {
     expect(body.token_endpoint_auth_method).toBe('none')
   })
 
+  it('accepts the three redirect_uris Cursor registers in one request', async () => {
+    // Cursor sends the legacy deeplink, the Cloud Agents web fallback and the
+    // RFC 8252 loopback together; one unknown URI used to fail the whole set.
+    const uris = [
+      'cursor://anysphere.cursor-mcp/oauth/callback',
+      'https://www.cursor.com/agents/mcp/oauth/callback',
+      'http://localhost:8787/callback',
+    ]
+    const response = await POST(createRequest({
+      client_name: 'Cursor',
+      redirect_uris: uris,
+      token_endpoint_auth_method: 'none',
+    }))
+    expect(response.status).toBe(201)
+    const body = await response.json()
+    expect(body.redirect_uris).toEqual(uris)
+  })
+
+  it('rejects other cursor.com paths and other cursor:// authorities', async () => {
+    for (const uri of [
+      'https://www.cursor.com/agents/mcp/oauth/callback2',
+      'https://cursor.com/agents/mcp/oauth/callback',
+      'cursor://evil.extension/oauth/callback',
+    ]) {
+      const response = await POST(createRequest({ redirect_uris: [uri] }))
+      expect(response.status, uri).toBe(400)
+    }
+  })
+
   it('rejects other grok.com paths', async () => {
     const response = await POST(createRequest({
       redirect_uris: ['https://grok.com/oauth/callback'],