fix(firestore): populate last_update_time in list_sessions from Firestore updateTime

[nileshpatil6]

Fixes #5632

Problem

list_sessions() hardcodes last_update_time=0.0 in every returned Session, silently ignoring the updateTime field stored in the Firestore document. This means callers can never sort or filter sessions by last-update time when using FirestoreSessionService.

get_session() already handles this correctly:

# Convert timestamp
update_time = data.get("updateTime")
last_update_time = 0.0
if update_time:
    if isinstance(update_time, datetime):
        last_update_time = update_time.timestamp()
    else:
        try:
            last_update_time = float(update_time)
        except (ValueError, TypeError):
            pass

list_sessions() was missing this block entirely.

Fix

Apply the same timestamp conversion in the list_sessions() loop, immediately before constructing each Session. The datetime import is already present at the top of the file.

Behavior

• Before: session.last_update_time is always 0.0 for every session returned by list_sessions()
• After: session.last_update_time reflects the actual Firestore updateTime (a POSIX timestamp float)

No other behavior changes -- the fix is additive and cannot break existing callers since 0.0 was always wrong.

[kkj333]

Thanks for working on this! This seems to overlap with #5640 (opened earlier),
which includes the same fix and unit tests.
Could we consolidate into one PR to avoid duplication?

[nileshpatil6]

Thanks for the pointer @kkj333 — closing this in favour of #5640 which was opened first and already includes unit tests. Good to see this getting fixed!