fix(mcp): mark sqlite_integrity not-applicable on non-chroma backends (#1931)

[mvalentsev]

Fixes #1931

What does this PR do?

On a non-chroma backend (qdrant, pgvector, sqlite_exact, sqlite_vec) the MCP
mempalace_status tool reported a passing SQLite integrity check that never ran:

"sqlite_integrity": { "checked": true, "ok": true,
                      "sqlite_path": ".../chroma.sqlite3",
                      "error_count": 0, "errors": [] }

_refresh_sqlite_integrity_status() already short-circuits the check for
non-chroma backends, and chroma.sqlite3 does not exist for the active backend,
so this reads as "integrity verified OK" when nothing was verified and points at
a file that is not there.

_sqlite_integrity_payload() now detects a non-chroma backend and reports the
check as not-applicable:

"sqlite_integrity": { "checked": false, "ok": null, "sqlite_path": "",
                      "error_count": 0, "errors": [],
                      "reason": "chroma.sqlite3 integrity check does not run for backend 'qdrant'" }

Recorded errors only ever come from the chroma path, so a genuine corruption
report is still surfaced unchanged; only the clean non-chroma case is
reclassified. The chroma payload keeps the same keys, so existing clients are
unaffected, and backend resolution is wrapped so status output cannot raise.

Scope is limited to the status payload. repair.sqlite_integrity_errors() is
reached only behind existing chroma guards, so it is left unchanged.

How to test

uv run pytest tests/test_mcp_server.py -k sqlite_integrity

New tests cover the non-chroma not-applicable payload, an unresolved-backend
fallback, and a regression guard that the chroma payload is unchanged.

Manual check on any non-chroma palace (for example MEMPALACE_BACKEND=sqlite_exact):
mempalace_status now returns sqlite_integrity.checked == false and
ok == null with a reason, instead of true/true referencing a
non-existent chroma.sqlite3.

Checklist

• Tests pass (python -m pytest tests/ -v)
• No hardcoded paths
• Linter passes (ruff check .)