fix(ci): allow transitive prereleases in FastMCP prerelease job

[naji247]

Summary

• The nightly MCP Prerelease Compatibility job has failed every night since FastMCP 3.4.0b1 landed on PyPI. The failure is in the install step, not a test — the test-mcp-prerelease job passes all tests; only the FastMCP job breaks.
• Root cause: FastMCP 3.4.0 restructured into fastmcp + a transitive fastmcp-slim package. Pinning fastmcp==3.4.0b1 drags in a prerelease fastmcp-slim==3.4.0b1 that isn't named in our pyproject.toml, so uv sync --prerelease if-necessary-or-explicit refuses it and resolution fails (No solution found ... try: --prerelease=allow).
• Fix: switch that one install step to --prerelease allow, the correct setting for a prerelease-testing job. The stable mcp-compatibility.yml workflow is unaffected (it uses no --prerelease flags).

There is no SDK breaking change — the full suite passes against FastMCP 3.4.0b1. The 3.4 change is purely a packaging split.

Test plan

• Reproduced exact CI error locally: uv pip compile with if-necessary-or-explicit fails identically
• Confirmed --prerelease allow resolves: fastmcp 3.4.0b1, fastmcp-slim 3.4.0b1, mcp 1.28.0
• Ran the exact CI command uv sync --extra dev --extra community --prerelease allow on a sed-modified pyproject → exit 0
• Ran the full test suite against FastMCP 3.4.0b1 in an isolated venv → 436 passed, 4 skipped, 3 xfailed
• Trigger the workflow via workflow_dispatch to confirm the nightly job goes green