fix(server): respect include_router prefix when mounting MCP endpoints (adopted from #329, fixes #204)

[K4bain]

Adopted from #329 (original patch by @wakqasahmed) — rebased onto current main and verified. Fixes #204.

Why this is needed

The documented FastAPI pattern — define routes on a sub-router, then app.include_router(router, prefix=/api) — is broken for MCP mounting: mount_http()/mount_sse() registered the MCP endpoints under the app root because the include-time prefix was never recovered. Issue #204's exact repro (a pure mount-point router with zero routes of its own) is the worst case: there is nothing to diff paths against.

The fix resolves the include prefix in two tiers:

1. FastAPI >= 0.137: read FastAPI's own include-time bookkeeping (_IncludedRouter.original_router identity + include_context.prefix) — exact, works even for empty routers, immune to endpoint reuse across routers.
2. Older FastAPI: diff the router's routes against their included counterparts (same endpoint fn + methods + path suffix), requiring every route to agree before accepting a prefix.

Ambiguity (router included at multiple prefixes, shared endpoints on old FastAPI) falls back to the safe empty prefix rather than guessing wrong.

Verification

• 4 new tests (incl. the literal [BUG] router prefix is ignored #204 repro, shared-endpoint-across-routers, multi-prefix inclusion, and the plain prefixed-router case) — all fail on main, pass with the fix (FastAPI 0.141.1, metadata path).
• Full unit suite: 73 passed (69 existing + 4 new).

Credit to @wakqasahmed for the original report and patch in #329.

[K4bain]

Closing for housekeeping - the repo had no reviewer engagement and we are re-approaching these areas through a narrower pipeline. Happy to reopen on request.