Add field tracking tests for text input components

This adds new tests to prevent regression of focus/registration bugs in
text input components:

Registration tests:
- test_search_bar_registers: verify search bar registers buffer
- test_textfield_with_selection_registers: verify selection variant
- test_search_view_registers: verify search modal registers query

Focus persistence tests:
- test_search_bar_focus_persists: focus maintained across frames
- test_unregistered_field_loses_focus: negative test for tracking
- test_focus_switch_between_fields: multi-field focus transfer via click

State transition tests:
- test_disabled_field_clears_focus: disabled clears focus
- test_read_only_field_clears_focus: read_only clears focus
- test_slider_unrendered_clears_active: slider state cleanup
- test_rerender_after_skip_frame: conditional rendering behavior

Stress test:
- test_many_textfields_stress: 32 fields to test hash collisions

These tests would have caught the bug fixed in commit 41c4b76 where
text input components failed to register with field tracking system.

All focus tests use real mouse input simulation to test actual focus
handling logic, not direct state manipulation.

Author: jserv