Add day-of-week validation for task dates

[partymola]

When dueDate is set on create_task or update_task, the caller must also pass expectedDayOfWeek (e.g. "Monday"). The server validates the date actually falls on that day and returns an error on mismatch.

Why

LLM agents sometimes say "Monday" but pass a date that is actually a Tuesday. This cross-check catches the inconsistency before the task is created.

Details

• Validates in the task's timeZone when provided, so late-night UTC dates resolve to the correct local day
• Error messages include the timezone used: "2026-04-14 is Tuesday in Europe/London, not Monday"
• Hardcoded English day names (locale-independent) - non-English names rejected with clear error listing valid values
• expectedDayOfWeek is excluded from TickTick API payloads (validation-only, never sent upstream)
• Works on both ticktick_create_task (function parameter) and ticktick_update_task (TaskObject field with exclude=True)

Test plan

• 16 unit tests covering: match/mismatch, case insensitivity, whitespace, timezone boundary crossing, non-English rejection, all 7 days, datetime object input, TaskObject serialization exclusion