fix(mcp): return structured compact results

[WarGloom]

What does this PR do?

Every MCP tool advertises an object outputSchema, but compact TOON and
plain-text results omitted structuredContent. Strict MCP clients therefore
rejected otherwise successful tool calls with -32600.

This change keeps JSON-object results unchanged and adds an object for every
other result:

• successful text and TOON payloads use {"text":"..."}
• error text uses {"error":"..."}

The existing text content block remains for model visibility and backwards
compatibility.

Validation

• ASAN_OPTIONS=detect_leaks=0 make -f Makefile.cbm test — 6,009 passed,
  1 skipped
• make -f Makefile.cbm lint-ci
• wire-level coverage for compact search_graph responses plus unit coverage
  for plain-text and error results

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]

Thanks for the compact structured-result fix. This is a normal-priority 0.9.1-rc MCP compatibility bug. Review will verify the JSON-RPC structuredContent contract, content fallback behavior for older clients, error responses, and that compact mode does not silently change existing tool payload semantics.

[DeusData]

Merged. This closes a real protocol-compliance gap in the TOON compact-output work (#1013): every MCP tool advertises an object outputSchema, but the compact-TOON and plain-text result paths omitted structuredContent, so strict clients rejected otherwise-successful calls with -32600. Verified: the three assertions are genuine guards (RED without your change — structuredContent absent — GREEN with it), the value is a proper {"text": ...} object (no double-encoding), JSON-object results are unchanged, mcp suite 165/165, lint clean. Thank you — good catch on the strict-client rejection.