fix(bluebubbles): use UTC epoch for tempGuid instead of naive local time

[webtecnica]

Summary

gateway/platforms/bluebubbles.py built temporary message GUIDs with datetime.utcnow().timestamp(). utcnow() returns a naive datetime, and calling .timestamp() on a naive datetime interprets it as local time, not UTC — so the embedded epoch was offset by the machine's UTC offset (e.g. −8h on UTC+8, −3h on UTC−3).

Root Cause

Two call sites (line ~513 _create_chat_for_handle and line ~569 chunked send path) used datetime.utcnow().timestamp(), producing a temp-<epoch> GUID whose timestamp is wrong by the local UTC offset.

Change

• gateway/platforms/bluebubbles.py: both call sites now use datetime.now(timezone.utc).timestamp() (correct UTC epoch); timezone added to the datetime import.
• tests/gateway/test_bluebubbles.py: new TestBlueBubblesTempGuid class asserting both outbound payloads embed a UTC epoch within a 5s tolerance of time.time() (a local-offset bug drifts by hours, far beyond the window).

Verification

• tests/gateway/test_bluebubbles.py: 20 passed (18 pre-existing + 2 new).

Closes #79511

[alt-glitch]

This was generated by AI during triage.

Duplicate of #23820: both replace datetime.utcnow().timestamp() with datetime.now(timezone.utc).timestamp() at the same two BlueBubbles tempGuid call sites.