Add core sync functions for identifier-based purgatory synchronization

Implement sync_identifier_next_url and sync_identifier_from_url functions
that provide the core URL selection and fetch logic for purgatory sync.

sync_identifier_next_url:
- Pure URL selection logic with no side effects
- Filters out our own domain and already-tried URLs
- Respects domain throttling when domain parameter is None
- Can target a specific domain when domain parameter is Some

sync_identifier_from_url:
- Fetches OIDs from a specific URL via the SyncContext
- Tracks request start/completion with ThrottleManager for rate limiting
- Calls process_newly_available_git_data on successful fetch

Also adds get_throttled_domains_with_untried_urls helper for the main
sync loop to know which DomainThrottle queues to enqueue identifiers to.

These functions are designed to be called by both:
- Main sync loop (tries non-throttled URLs immediately)
- DomainThrottle queue processing (when capacity frees up)

Includes 10 unit tests covering:
- Throttled domain skipping
- Tried URL skipping
- Our domain filtering
- Specific domain targeting
- Fetch success/failure handling
- Throttle request tracking

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