Widen #96038's fail-closed classifier to the gateway transcript retry
path: SessionStore._is_fts_corruption_error no longer treats a generic
'database disk image is malformed' as FTS-only damage. It now delegates
to SessionDB._is_fts_write_corruption_error (SQLITE_CORRUPT_VTAB result
code or explicit fts5 corrupt-structure text) and only keeps the
messages_fts-named cases. Structural corruption falls through to the
bounded retry/backoff path instead of rebuilding FTS and retrying writes
against a damaged database.
Sibling site spotted in PR #98090 by @fangliquanflq.
