refactor(): Replace SelectorFactory mixin with dedicated Selector classes

[benflexcompute]

…

Unified the entity selector API by replacing the SelectorFactory mixin pattern on entity classes (Surface, Edge, GenericVolume, GeometryBodyGroup) with dedicated Selector classes (SurfaceSelector, EdgeSelector, VolumeSelector, BodyGroupSelector).

Key changes:

• Removed SelectorFactory mixin inheritance from entity classes
• Created four new Selector classes that inherit from EntitySelector
• Each class has target_class locked as a frozen field (type-safe)
• Exported selector factories to fl namespace (fl.SurfaceSelector, etc.)
• Hid syntax parameter from fluent API (UI doesn't support regex)
• Updated docstrings to clarify "pattern-based selector" concept

Benefits:

• Clearer separation between entity access (draft.surfaces[...]) and selector definition (fl.SurfaceSelector(...))
• Better type safety with frozen target_class field
• More discoverable API without exposing entity classes
• Prevents accidental entity construction
• Enables reusable SimulationParams templates across geometries

This change maintains backward compatibility at the selector behavior level while improving the API ergonomics and design clarity.

All tests updated and passing:

• test_entity_selector_fluent_api: 6 tests
• test_entity_expansion_impl: 3 tests
• test_entity_list: 5 tests
• test_validators_params: 2 tests
• test_entity_processing_service: 7 tests
• test_selector_expansion_validators: 4 tests
• test_draft_context_end_to_end_submit_roundtrip: 1 test
• test_solver_translator: 8 tests