Summary

• keep SessionDB usable when the bundled SQLite build does not provide FTS5
• disable only full-text search in that case instead of dropping the entire SQLite session store
• add regression coverage for a simulated no-FTS5 SQLite runtime

Why

• some Python/SQLite builds do not ship the fts5 module, which currently makes gateway session storage fall back to JSONL entirely
• the store can still handle normal session/message persistence without FTS5, so failing the whole initialization is unnecessary

Fixes #10897

Testing

• pytest -o addopts='' tests/test_hermes_state.py -k 'db_initializes_without_fts5_module or search_messages'
• pytest -o addopts='' tests/test_hermes_state.py