fix(web): expose chat variable tips state

[lyzno1]

Summary

• give the existing tips disclosure the stable accessible name TIPS
• expose its open state with aria-expanded
• hide the decorative book glyph from the accessibility tree
• assert the real name and expanded state before and after activation

Dependency

Stacked on #40333 because both layers update the same owner test and header. This layer does not depend on the close action at runtime and remains a single semantic contract.

Validation

• pnpm --dir web exec vp test run app/components/workflow/panel/chat-variable-panel/__tests__/index.spec.tsx — 4/4 passed
• pnpm --dir web lint:a11y 'app/components/workflow/panel/chat-variable-panel/index.tsx' — passed
• pnpm check — 0 errors / 2059 existing warnings
• git diff --check — passed
• the strengthened name/expanded assertion failed against the old unnamed control before the production change

Visual regression review

Expected visual delta: none.

Please verify:

• the book button keeps the same 24×24 ActionButton box and 16×16 glyph
• active, hover, collapsed, expanded, light-mode, and dark-mode visuals are unchanged
• showing or hiding the tips area preserves the existing panel layout transition and spacing
• the close control remains aligned with the tips control

Only ARIA attributes were added; the existing ActionButtonState.Active remains the sole visual-state owner.

Rollback

Revert this PR without reverting #40333. The close action remains valid independently.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.43%. Comparing base (2c29d75) to head (e8c95f2).

Additional details and impacted files

@@                          Coverage Diff                          @@
##           codex/a11y-chat-variable-panel-close   #40334   +/-   ##
=====================================================================
  Coverage                                 86.42%   86.43%           
=====================================================================
  Files                                      5107     5107           
  Lines                                    290741   290741           
  Branches                                  57924    57924           
=====================================================================
+ Hits                                     251287   251290    +3     
+ Misses                                    34513    34510    -3     
  Partials                                   4941     4941           

Flag	Coverage Δ
dify-ui	93.49% <ø> (ø)
web	86.11% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.