[MCP T8] search_code MCP tool

[DvirDukhan]

Phase 1 ticket T8. Depends on #652 (T4 index_repo).

Context

search_code is the agent's entry point for finding symbols by name when it doesn't yet have a symbol id. It's prefix search across the Searchable label, backed by FalkorDB's full-text index. The underlying operation already exists: Graph.prefix_search() (api/graph.py:325-358), AsyncGraphQuery.prefix_search() (api/graph.py:714-724), and async_prefix_search() (api/auto_complete.py:10-16). The CLI command cgraph search (api/cli.py:265-282) already exposes it.

Scope

In:

• api/mcp/tools/structural.py::search_code(prefix, limit=20).
• Wraps async_prefix_search() (api/auto_complete.py:10-16) or AsyncGraphQuery.prefix_search() (api/graph.py:714-724) — whichever produces the cleanest call site.
• Returns a typed result list:

  [
    {"id":"...","name":"foo","label":"Function","file":"src/x.py","line":12},
    ...
  ]

• Tests in tests/mcp/test_search_code.py:
  • Unit (mocked graph): empty result, single result, limit honored.
  • Integration: assert known prefix hits against T3 fixture (e.g. search_code("ent") returns a node with name entrypoint).
  • CLI parity vs cgraph search: same prefix → same hits.
  • Protocol round-trip.

Out:

• Full-text / fuzzy search (only prefix in v1).
• Embedding / semantic search.
• Filter by label, file, or directory (could be added later via params).

Files to create / modify

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

Acceptance criteria

• Tool registered with FastMCP.
• Input schema: prefix: str, limit: int = 20.
• Unit tests cover empty, single, and limit-honored cases.
• Integration test against T3 fixture: at least one named-prefix assertion from expected.yaml passes.
• CLI parity test: cgraph search <prefix> and the MCP tool return the same set of node ids for the same prefix.
• 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)

• Fuzzy / typo-tolerant search.
• Semantic / embedding search.
• Label or path filters (deferred — could be added via optional params).

Notes for the implementer

• The wrapper should be ~15 lines.
• limit should be enforced server-side via the underlying prefix_search limit parameter — don't slice in Python after fetching everything.
• Reuse the protocol-test helper from T4.