fix(session/database): auto-populate update_time for app/user state rows (#1177)

[rootkiller6788]

What

Fixes #1177 — persisting app- or user-scoped state via session/database fails on MySQL with default strict mode:

Error 1292 (22007): Incorrect datetime value: '0000-00-00' for column 'update_time' at row 1

storageAppState / storageUserState declare UpdateTime without any auto-update tag, and neither write path (Create, AppendEvent) assigns it, so GORM writes Go's zero time.Time explicitly. MySQL's NO_ZERO_DATE/STRICT_TRANS_TABLES rejects that value. SQLite, PostgreSQL and Spanner happen to accept it, which is why the bug only shows up on MySQL.

Fix

Tag both state models with autoUpdateTime (mirroring adk-python, whose StorageAppState.update_time uses default=func.now(), onupdate=func.now()):

UpdateTime time.Time `gorm:"precision:6;autoUpdateTime:micro"`

GORM then fills the column with the current time on both INSERT (new app/user row) and UPDATE (state delta on an existing row), covering every save path instead of just the two call sites. precision:6 is preserved, so the DDL is unchanged (datetime(6)).

Testing

• New regression test TestDatabaseService_StateTablesPopulateUpdateTime verifies update_time is populated after Create (app + user state) and refreshed after AppendEvent with a state delta. Confirmed it fails before the fix (zero update_time) and passes after.
• go test ./session/database/... — all green.
• go test ./session/... — session, session/database, session/sessiontestsuite green; session/vertexai is not runnable in this environment (requires live gRPC/Vertex AI network access).
• go build -mod=readonly work — passes.
• go mod tidy -diff — no changes.

A note on -race: the race detector fails to start in this Windows sandbox (exit status 0xc0000139, missing runtime entry point), so -race runs are not available here; the change is limited to GORM struct tags and a test, with no new concurrency.

[wolo-lab]

Thanks for the fix. It's solved by #1204.