MCP fetch has no timeout, a hung registry hangs the Worker

[royalpinto007]

Problem

A hung registry hangs the Worker: the MCP fetch has no signal/AbortSignal.timeout, so a never-resolving upstream pends forever.

Where

src/worker/mcp.ts (bare fetch; sibling Casebook-MCP/src/data.ts already uses AbortSignal.timeout(5000)).

Acceptance

Add a failing test first with a never-resolving fetch, expecting rejection/"Registry unreachable" within ~5s instead of pending forever. Then fix.