Fix inconsistent ISO timestamp formatting causing ValueError on step update

[hztBUAA]

Summary

Fixes #2491

ChainlitDataLayer saves timestamps with a trailing Z (e.g., 2025-09-04T02:00:42.164000Z) via ISO_FORMAT, but reads them back using Python's .isoformat() which omits the Z (e.g., 2025-09-04T02:00:42.164000). When update_step calls create_step with the read-back createdAt string, strptime fails with:

ValueError: time data '2025-09-04T02:00:42.164000' does not match format '%Y-%m-%dT%H:%M:%S.%fZ'

Changes

• Add _parse_iso_datetime() helper that tolerates both with and without trailing Z
• Add _datetime_to_utc_iso() helper that ensures all datetime-to-string conversions consistently include the trailing Z
• Replace all bare .isoformat() calls with _datetime_to_utc_iso() across user, thread, and step serialization
• Replace datetime.strptime(created_at, ISO_FORMAT) with _parse_iso_datetime(created_at) in create_step
• Switch datetime.now() to datetime.utcnow() in get_current_timestamp() and update_thread() for correct UTC semantics
• Add comprehensive tests covering parse/format roundtrip and step row conversion

Test plan

• _parse_iso_datetime correctly parses both "...Z" and "..." formats
• _datetime_to_utc_iso always produces strings ending with Z
• Step row conversion produces Z-suffixed timestamps that can be re-parsed by create_step
• None timestamps are preserved as None (not formatted)
• Invalid formats still raise ValueError

---

Summary by cubic

Fix inconsistent ISO timestamp handling to prevent ValueError during step updates. Standardizes UTC timestamps with a trailing Z and makes parsing accept both formats.

• Bug Fixes
  • Added tolerant ISO datetime parsing (with or without trailing Z).
  • Added UTC formatting helper that always appends Z; replaced isoformat/strptime usages across user, thread, and step serialization.
  • Switched to datetime.utcnow for current timestamps and thread updates.
  • Made element fields optional-safe and guarded storage read URL retrieval.
  • Added tests for parse/format roundtrip and step timestamp conversion.

^{Written for commit 9b70738. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files

[dokterbob]

Would love to merge this, please run uv run ruff format over your code! :)