fix(tools): only require a timezone where one is actually needed

[marcelklehr]

Follow-up to #257, from a post-merge review.

Fixes

• update_scheduled_task overwrote the task's timezone. Every update sent timezone (falling back to the profile timezone), so a prompt-only edit of a task stored in Asia/Tokyo silently moved it to the user's profile zone. For users without a profile timezone, every update raised. It now sends a timezone only when one is passed explicitly, as before fix(tools): require timezone on time tools, inject user timezone into the prompt #257. A naive starts_at is still interpreted in the explicit timezone, or the profile timezone otherwise.
• create_scheduled_task failed for users without a profile timezone, even when starts_at was omitted. starts_at_timestamp now only resolves a timezone for a starts_at without a UTC offset, so the error only fires when a timezone is actually needed.
• update_task swallowed the new timezone error. get_timezone() was called inside the bare except: continue, so an invalid or missing timezone came back as False ("task not found"). The timezone is now resolved before the loop.
• The starts_at docs still used the example 2025-01-01T09:00:00Z after fix(tools): require timezone on time tools, inject user timezone into the prompt #257 removed "use the user's timezone". Since offset-aware values ignore timezone, this pushed the model to send local times as UTC. The example is now naive local time.

Tests

tests/test_timezone_resolution.py, which runs standalone or under pytest. 5 of its 11 tests fail on current main and all pass here. tests/test_schedule_event_payload.py still passes.

Other findings from the review, not addressed here

• user_timezone_async caches a failed lookup (None) per user for 300s (lib/tz.py).
• When the user has no email, schedule_event writes the organizer as mailto: (calendar.py).
• All-day events now use DTEND = end_date + 1, which adds a day for callers that already pass an exclusive end date.
• schedule_event_sync fetches /cloud/user twice per event. The first fetch has no retry.
• Timezone validation is written three ways with different fallbacks (agent.py, datetime_info.py, calendar.py).
• tests/test_schedule_event_payload.py replaces cal.NextcloudApp without restoring it, and has an unused _FakeNc class. assignments.py has an unused pytz import.

🤖 Generated with Claude Code

[WSHAPER]

LGTM, however one judgment call to confirm: empty-string timezone now raises where main fell back to the profile. I think the raise is right per the documented contract, but it's a small behavior change beyond the fixes and has no test. Wdyt?

[marcelklehr]

I think the raise is right per the documented contract

i agree