CalDAV adapter

[SashankBhamidi]

src/calendaring_sync/adapters/caldav.py:

CalDAVAdapter(calendar, state_store, cache)
    .sync(resolver_fn=None) -> SyncResult
    .push(url, ical_content)
    .delete(url)

SyncResult: lists of applied, conflicted, and failed changes.

Calls python-caldav's calendar.sync() to get changed objects since the last sync-token. Feeds each change through the state store and conflict detector. Updates the cache after a successful sync.

Optional dependency: python-caldav>=3.0. If not installed, importing this module raises ImportError with a clear message pointing to the install extra.

Unit tests mock python-caldav's sync() response. Integration test uses Radicale (embedded, no Docker needed).

Scenarios to cover:

• First sync populates cache
• Second sync with server-side change classifies as REMOTE_ONLY
• Local change classifies as LOCAL_ONLY
• Conflict surfaces as ConflictError, not dropped silently