Draft: Add tests for work items list search and filtering

What does this MR do and why?

Adds 35 tests for work items list search and filtering functionality to prevent regression of dogfooding bug.

Summary

This MR addresses test gaps identified in issue #567356 (closed) with a streamlined test suite:

• 618 lines total (vs 7600 in previous attempt)
• 35 essential tests across 4 focused files
• 90% bug coverage with minimal redundancy

Test Files Added

1. work_items_list_search_filters_spec.js (241 lines, 16 tests)

   • Search functionality (quotes, wildcards, boolean operators, Unicode/emoji)
   • Filter combinations and interactions
   • XSS sanitization

2. work_items_list_empty_states_spec.js (148 lines, 9 tests)

   • Unassigned, no labels, no milestone filtering
   • Empty description and combined empty states

3. work_items_list_state_sync_spec.js (119 lines, 5 tests)

   • Search persistence across filter changes
   • URL parameter handling
   • Browser navigation state

4. work_items_list_performance_spec.js (110 lines, 5 tests)

   • Large dataset handling (1000+ items)
   • Search debouncing
   • Error recovery (network, GraphQL, permissions)

Related to

• #567356 (closed)