[MCP T7] find_path MCP tool

[DvirDukhan]

Phase 1 ticket T7. Depends on #652 (T4 index_repo).

Context

find_path answers "how does symbol A reach symbol B?" — useful for understanding indirect call chains and refactoring sequences. The underlying operation already exists: Graph.find_paths() (api/graph.py:535-579) and AsyncGraphQuery.find_paths() (api/graph.py:726-745). The CLI command cgraph paths (api/cli.py:317-335) already exposes it. This ticket is a thin async wrapper.

Scope

In:

• api/mcp/tools/structural.py::find_path(source_id, dest_id, max_paths=10).
• Wraps AsyncGraphQuery.find_paths() (api/graph.py:726-745).
• Returns a list of node-sequences, each representing a call path:

  [
    {"path": [{"id":"a","name":"a","label":"Function","file":"...","line":1}, ...]},
    ...
  ]

• Tests in tests/mcp/test_find_path.py:
  • Unit (mocked graph): assert path serialization shape.
  • Integration: known path against T3 fixture (e.g. entrypoint → service → repo → db) — assert the path exists in the result and has the expected length.
  • No-path case: source and dest with no connecting path returns [], not an error.
  • CLI parity vs cgraph paths: same input → same output.
  • Protocol round-trip.

Out:

• Path ranking / shortest-path-only mode (deferred).
• Bidirectional path search.
• Cross-relation paths beyond the existing find_paths Cypher (CALLS* only).

Files to create / modify

• api/mcp/tools/structural.py — tool registration
• new tests/mcp/test_find_path.py

Acceptance criteria

• Tool registered with FastMCP.
• Input schema: source_id: str, dest_id: str, max_paths: int = 10.
• Unit test asserts response shape with mocked graph.
• Integration test against T3 fixture: find_path(entrypoint, db) returns at least one path that traverses entrypoint → service → repo → db.
• No-path case (e.g. two disjoint nodes) returns [] — not an exception, not a 404.
• CLI parity test: cgraph paths <src> <dst> and the MCP tool return the same paths for the same input.
• Protocol round-trip test.
• CI workflow [MCP T2] CI workflow with FalkorDB service for MCP tests #649 green.

Dependencies

• Blocks: nothing.
• Blocked by: [MCP T4] index_repo MCP tool #652 (T4).

Out of scope (do NOT do in this PR)

• Shortest-path-only mode (the existing find_paths returns all paths up to a depth limit).
• Bidirectional search.
• Cross-relation paths (only CALLS per the existing implementation).

Notes for the implementer

• The wrapper should be ~20 lines. Real work is in api/graph.py.
• max_paths is a safety cap to prevent the agent from blowing the response size on highly-connected graphs. If find_paths already supports a limit param, forward it; otherwise add one.
• Reuse the protocol-test helper from T4.