fix(mcp): isolate semantic-only JSON search

[JJordan0C]

What does this PR do?

Fixes #1295.

search_graph already skipped the structural query for semantic-only calls in
the default response format, but format:"json" still ran an unfiltered
structural search. On a 21,221-node index that made the JSON response report all
21,221 nodes and include unrelated groups alongside the semantic matches.

This change shares the structural-filter detection between both output paths
and skips cbm_store_search for semantic-only JSON calls. Mixed
semantic/structural searches keep their existing behavior.

The regression test covers both output formats. The end-to-end reproduction now
returns total: 0, count: 0, and no structural groups while retaining
semantic rows.

Validation:

• Full test runner: 6,792 passed, 4 platform-only skips
• Full ASan/UBSan test runner: 6,792 passed, 4 platform-only skips
• CI lint gate: cppcheck 2.20.0, clang-format 20, NOLINT policy
• Production binary reproduction against the repository's full index

Checklist

• Every commit is signed off (git commit -s) — required, CI rejects
  unsigned commits (DCO, see CONTRIBUTING.md)
• Tests pass locally (make -f Makefile.cbm test)
• Lint passes (make -f Makefile.cbm lint-ci)
• New behavior is covered by a test (reproduce-first for bug fixes)

[DeusData]

Thank you for the contribution and for covering the semantic-only JSON path with end-to-end regression tests. This is now triaged as a high-priority MCP output correctness bug for 0.9.1-rc. Current main already contains an overlapping fix and regression coverage for #1295, so review will determine whether this branch still contains any distinct work. Our community PR queue is currently quite full, so it may take a little time before we can complete that review. We are doing our best to support community contributions and will return with code-grounded feedback as capacity opens.