fix: rewrite bare /mcp to /mcp/ in MCPPathRewriteMiddleware so stream…

[omorros]

🔗 Related Issue

Closes #4275

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📝 Summary

MCPPathRewriteMiddleware only rewrote /servers/<id>/mcp to /mcp/, leaving bare /mcp untouched. Because the Starlette mount is at
/mcp (compiled as /mcp/{path:path}), /mcp without the trailing slash did not match and FastAPI's router emitted a 307 redirect to
/mcp/. For chunked Streamable HTTP POSTs the client cannot replay the body across a redirect, so the handshake failed with httpx.ReadError
during initialize.

This PR normalises bare /mcp to /mcp/ inside the middleware so the mount matches directly without a redirect. It also mirrors every
rewrite into scope["raw_path"] alongside scope["path"] (both the new bare-/mcp branch and the existing /servers/<id>/mcp branch),
keeping downstream URL reconstruction consistent with the rewritten path.

Security posture is unchanged: arbitrary prefixes ending in /mcp (e.g. /foo/mcp) still pass through unrewritten, and empty server IDs
(/servers//mcp) still return 404.

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🏷️ Type of Change

• Bug fix
• Feature / Enhancement
• Documentation
• Refactor
• Chore (deps, CI, tooling)
• Other (describe below)

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🧪 Verification

Check	Command	Status
Lint suite	make lint	✅ ruff/isort/black clean on changed lines (pre-existing drift untouched)
Unit tests	make test	✅ 1864+ related tests pass (main, middleware, auth, streamable HTTP transport)
Coverage ≥ 80%	make coverage	⏳ to be verified in CI

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✅ Checklist

• Code formatted (make black isort pre-commit)
• Tests added/updated for changes
• Documentation updated (if applicable)
• No secrets or credentials committed

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📓 Notes (optional)

Four new unit tests in TestMCPPathRewriteMiddleware cover:

• test_rewrite_bare_mcp_to_trailing_slash — /mcp → /mcp/
• test_rewrite_bare_mcp_with_root_path — /gateway/mcp → /gateway/mcp/ (reverse-proxy prefix)
• test_rewrite_updates_raw_path — /servers/<id>/mcp rewrite keeps raw_path aligned
• test_bare_mcp_updates_raw_path — bare-/mcp rewrite syncs raw_path