feat(calendar): expose ordered VALARM reminders

[silviuchingaru]

Summary

• Adds ordered VALARM reminder parsing/serialization for calendar events and todos.
• Supports relative reminders, absolute trigger_at reminders, ordered list updates, and explicit clearing with reminders: [].
• Preserves existing VALARMs when unrelated event/todo fields are updated.

Test Plan

• uv run ruff check nextcloud_mcp_server tests/unit/client/test_calendar.py
• uv run pytest tests/unit/client/test_calendar.py -q
• Live Nextcloud smoke test against a temporary calendar: event/todo VALARM create, preserve, update, clear; cleanup returned 204s.

[CLAassistant]

All committers have signed the CLA.

[sonarqubecloud]

Quality Gate failed

Failed conditions
C Security Rating on New Code (required ≥ A)

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