test: add 226 tests covering all 52 Rust handler classes

Files changed:
- tests/test_rust_handler_coverage.py (NEW) — 226 tests in 4 batches
- CHANGELOG.md — Added test coverage expansion entry
- .pipeline-state/test-coverage-expansion-plan.md (NEW) — Planning doc

Batch 1 (Rendering): 33 previously untested handlers tested for HTML output,
  CSS classes, and attribute handling — CardHandler, TabsHandler, AccordionHandler,
  AccordionItemHandler, DropdownHandler, AlertHandler, FormGroupHandler,
  TimelineHandler, TimelineItemHandler, TooltipHandler, ToastContainerHandler,
  ProgressHandler, BadgeHandler, PaginationHandler, AvatarHandler, SpinnerHandler,
  SkeletonHandler, BreadcrumbHandler, EmptyStateHandler, DividerHandler,
  SwitchHandler, StatCardHandler, TagChipHandler, StepperHandler, DjButtonHandler,
  DjInputHandler, DjSelectHandler, DjTextareaHandler, DjCheckboxHandler,
  DjRadioHandler, DataTableHandler, plus 17 delegating handlers

Batch 2 (Form Interaction): 12 tests verifying dj-input, dj-change, dj-click
  event attribute emission for all form and interactive handlers

Batch 3 (Complex State): 15 tests for DataTable sort indicators/selection/
  pagination/search, Stepper active/complete states, Breadcrumb active items

Batch 4 (Edge Cases): 26 empty-data tests + 43 XSS payload tests covering
  every handler that accepts user-supplied text

Test results: 817 passed (591 existing + 226 new), 0 failed
