fix(search): preserve compound requested fields

[Enferlain]

What does this PR do?

Fixes #1284.

Requested extra fields in search_graph were serialized only for scalar JSON values. Stored arrays and objects fell through to an empty cell in table output and were silently misrepresented.

This change centralizes requested-field serialization: table output emits compact JSON text for compound values, while JSON output copies the original JSON value so its type is preserved. Regression coverage verifies both output formats.

Verification

• make -f Makefile.cbm test-focused TEST_SUITES='cypher extraction registry pipeline mcp index_resilience'
• Result: 884 passed, 2 skipped

Checklist

• Every commit is signed off (git commit -s) and follows the Contributor License Agreement
• Full test suite passes locally (make -f Makefile.cbm test) — focused sanitized suites passed
• Lint passes (make -f Makefile.cbm lint-ci) — not run locally
• New behavior is covered by regression tests that would fail without this fix