Fix update_task wiping unrelated fields on partial updates

[partymola]

Problem

update_task silently resets fields that were not included in the update call. Three distinct bugs combine to cause this:

1. model_dump() includes defaults for unset fields

TaskObject declares priority with a default of 0, so calling update_task with only a new title still sends priority=0 to the API, resetting any existing priority. The same applies to every Optional field that defaults to None -- dates, reminders, content, etc. are all sent as null.

2. The merge result is never used

Lines 312-313 fetch the existing task and call task_obj.update(task_object), intending to merge, but line 315 sends task_object.model_dump() (the raw partial input) instead of the merged task_obj. The merge has no effect.

3. dict.update() with a Pydantic model doesn't work correctly

task_obj is a dict (from get_by_id), but task_object is a Pydantic TaskObject. dict.update() iterates the model's attribute names as keys, which does not produce a correct field-level merge.

Failure scenarios

• User sets priority=5 on a task with dates and reminders. Later calls update_task to change only the title. The task loses its priority (reset to 0), its dates, and its reminders.
• User calls update_task to reschedule a task to a new date. The task's priority is reset to 0 and its reminders are cleared.
• Any partial update effectively becomes a full replace with defaults for all unspecified fields.

Additionally, sending the full API-response dict back (which includes read-only fields like creator, deleted, kind, isFloating) causes HTTP 500 errors from the TickTick API. And reminders returned as objects [{id, trigger}] must be normalized to strings before sending back.

Fix

• Use model_dump(exclude_unset=True) so only explicitly provided fields are included in the update payload
• Merge those fields into the existing task fetched via get_by_id()
• Filter the merged dict to only API-accepted fields (UPDATABLE_FIELDS whitelist)
• Normalize reminders from object format [{id, trigger}] to string format ["TRIGGER:..."]

Tests

10 unit tests added covering:

• Partial update (only priority) preserves existing dates, reminders, title, tags
• Partial update (only dates) preserves existing priority
• Reminders in object format are normalized to strings
• Mixed object/string reminders are handled correctly
• Read-only fields (creator, deleted, kind, isFloating, etag, etc.) are filtered out
• Default priority=0 is NOT sent when unset (preserves existing priority)
• Explicitly set priority=0 IS sent (overwrites existing)