Fix search score and threshold handling

[wayyoungboy]

What changed

• Preserve valid score=0.0 values when converting HTTP search results instead of turning them into null.
• Add threshold support to HTTP search requests for both POST body and GET query parameters.
• Pass the HTTP threshold value through SearchService to Memory.search().
• Add focused unit coverage for score conversion and threshold validation/propagation.

Why

Search results can legitimately have a zero score. The API converter previously used truthiness fallback, so 0.0 was treated as missing and serialized as null when no similarity fallback existed.

HTTP users also had no way to pass the existing core search threshold even though CLI and MCP already expose similar behavior.

Validation

PYTHONPATH=src .venv/bin/python -m pytest tests/unit/server/test_search_score_threshold.py -q

Result: 7 passed.