fix(mcp): trace_path cross_service follows CROSS_* edges (#522)

[kirilklein]

What does this PR do?

Fixes #522 — trace_path with mode=cross_service did not follow cross-repo edges, so cross-service traces silently dropped every cross hop even when the edge existed and was visible via query_graph.

Root cause. resolve_trace_edge_types() in src/mcp/mcp.c resolved the cross_service default edge-type list to {HTTP_CALLS, ASYNC_CALLS, DATA_FLOWS, CALLS} — none of the CROSS_* types the cross-repo pass actually writes. cbm_store_bfs filters edges with an exact WHERE e.type IN (...) clause, so any CROSS_HTTP_CALLS / CROSS_ASYNC_CALLS / … hop was filtered out. The traversal engine was fine; the defaults list was just missing the cross types.

Fix. Add the six CROSS_* types (CROSS_HTTP_CALLS, CROSS_ASYNC_CALLS, CROSS_CHANNEL, CROSS_GRPC_CALLS, CROSS_GRAPHQL_CALLS, CROSS_TRPC_CALLS) to the cross_service default list — exactly the set pass_cross_repo.c emits and cross_repo_delete_edges() cleans up. The count is now derived from the array size (sizeof) so it can't drift; the now-dead MCP_N_DEFAULTS_4 constant is removed. The trace_path tool description is updated to match.

Before / after (repro from the issue)

• Before: query_graph returns the CROSS_HTTP_CALLS edge, but trace_path(mode=cross_service) from the source function returns only in-repo callees.
• After: trace_path(mode=cross_service) surfaces the cross hop; query_graph still returns the edge.

Test

Adds integ_mcp_trace_path_cross_service: seeds a CROSS_HTTP_CALLS edge between two indexed functions that have no CALLS relationship, then asserts cross_service surfaces the hop while calls mode does not (proving the cross edge specifically is what's followed). Verified to fail before the fix and pass after.

Checklist

• Every commit is signed off (git commit -s)
• Tests pass locally (make -f Makefile.cbm test) — 5609 passed
• Lint passes — clang-format --dry-run --Werror clean on changed files (full lint-ci deferred to CI)
• New behavior is covered by a test (reproduce-first)

🤖 Generated with Claude Code

[azuttre]

Built this branch on macOS (arm64) and tested it: with a node reachable only through a CROSS_HTTP_CALLS edge, trace_path cross_service returns 0 callees on main and 1 on this branch. Confirmed working. Thanks for the quick turnaround.

[prakashelango]

I have a kinesis stream for eventing and it couldn't recognize the cross service calls from upstream and downstream, by any chance will this change include the kinesis too?