DEFAULT_DB_PATH in hermes_state.py is computed at import time, freezing
the developer's real ~/.hermes even when a test fixture (or runtime
profile switch) later redirects HERMES_HOME. Any default SessionDB() —
e.g. gateway SessionStore — then opened the real state.db.
Add _default_db_path(): resolves get_hermes_home() fresh at call time,
while a deliberately re-pointed DEFAULT_DB_PATH (the established
monkeypatch escape hatch) still wins via an import-time snapshot
comparison, preserving existing test behavior. SessionDB.__init__ and
session_search's requirement check now use the resolver; explicit
db_path arguments are untouched.
Reimplemented from PR #11875 by @JorkeyLiu (original diff predates the
hermes_state rewrite); regression test ported and modernized.
