Fix recurring task sync: Preserve RRULE semantics and fix update bugs

[deeqdev]

Summary

Fixed a bug where recurring tasks synced as one-time tasks or lost recurrence rules during updates.

Changes

• Extended Fields: Now preserves explicit machine-readable RRULE strings in Google Tasks metadata alongside the human-readable summary.
• Microsoft Provider:
  • Updated upsertTask to always send recurrence payload on PATCH (update) requests, ensuring one-time tasks can be converted to recurring and rule changes propagate.
  • Added fallback start date logic to deserializeRecurrence to prevent broken series when DTSTART is missing.
• Parsing: Improved deserializeRecurrence to handle WKST and TZID properly.

Tests

• Added test/extendedFields.test.ts to verify RRULE parsing/serialization round-trips.
• Updated test/googleProvider.test.ts to verify persistence of machine-readable recurrence.
• Updated test/microsoftProvider.test.ts to verify recurrence is sent on updates.
• Verified all tests pass, including web UI tests.