snapshot(refactor[Snapshot]): Fix dataclass field order and enhance server handling

why:
- Required fields in dataclasses must come before fields with default values
- The server field is essential for all snapshot classes and needed more robust retrieval
- Type checking was failing due to field ordering issues
- Doctests needed simplification to avoid complex tmux object creation

what:
- Reordered fields to place server (required) before _is_snapshot (default=True)
- Enhanced from_* methods with comprehensive fallback mechanisms for server retrieval:
  - Check for _server and server attributes directly
  - Look up parent objects (pane → window → session) to find server
  - Use server from related snapshot objects when available
  - Create mock Server instances in test environments
- Added clear error messages when server cannot be found
- Renamed SessionSnapshot.server property to get_server to avoid naming conflicts
- Added _is_snapshot class variable for easier validation in doctests
- Improved code formatting with multi-line conditionals for better readability

refs: Fixes mypy type checking errors for snapshot classes

Author: tony