feat(mcp): DingDuff connector support — OAuth redirect discovery and chat case retrieval

[duncanmcqueen]

Summary

Adds support for DingDuff as a user MCP connector, so the assistant can use DingDuff tools for case retrieval, case reading, and case-specific document lookup directly from chat.

Two changes make this work:

1. OAuth discovery behind redirects (backend/src/lib/mcp/client.ts, oauth.ts)

DingDuff's server 302s its well-known OAuth metadata to a path-scoped issuer, and the MCP SDK's own discovery does not follow redirects, so the OAuth flow could not complete.

• guardedFetch now follows redirects hop by hop instead of refusing them outright: every 3xx target is re-validated against the existing SSRF guard (HTTPS-only, private-IP/metadata-host blocks, DNS-pinned dispatcher), credentials are stripped on cross-origin hops (per the fetch spec), and redirected POSTs are converted to GET with the body dropped. Undici still never auto-follows (redirect: "manual" on every hop), so no redirect can smuggle egress past the guard.
• New discoverOAuthServerState plus the SDK's OAuthClientProvider.discoveryState hook hand the SDK pre-discovered metadata, so connectors whose well-known documents live behind a redirect complete the flow.

2. Chat integration (backend/src/lib/chat/streaming.ts)

When DingDuff MCP tools are present on a turn, the built-in CourtListener tools are withheld and the system prompt directs case retrieval at the DingDuff tools, so the two case-law sources are never mixed within a single turn.

Tests

• New cases in client.ssrf.test.ts: redirect following with per-hop re-validation, refusal to follow a redirect to a blocked address, credential stripping on cross-origin redirects, and POST→GET conversion.
• Full backend suite passes (554 tests); tsc --noEmit clean.

Notes

• No schema changes; no new dependencies.
• The legal-monitors DingDuff connector source is intentionally not part of this PR (that feature set isn't upstream).

[CLAassistant]

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✅ duncanmcqueen
❌ dwmcqueen
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[duncanmcqueen]

Being a lawyer a bit here (and did accept it), but the CLA says this:

9. Governing Law
   This Agreement shall be governed by and construed in accordance with the laws of [YOUR JURISDICTION, e.g., "the State of California, United States"], without regard to its conflict-of-law provisions.