test(dify-ui): remove brittle primitive assertions

[lyzno1]

Summary

• replace low-value Avatar tests with a real image loading status callback contract
• remove brittle ProgressCircle SVG path assertions and keep the test focused on progressbar semantics
• remove an unnecessary type assertion in the Avatar empty-string case

Testing

• pnpm -C packages/dify-ui exec vp test run --project unit src/avatar/tests/index.spec.tsx src/progress/tests/index.spec.tsx --silent=passed-only
• pnpm -C packages/dify-ui exec vp test run --project unit --silent=passed-only
• pnpm -C packages/dify-ui type-check
• pnpm eslint packages/dify-ui/src/avatar/tests/index.spec.tsx packages/dify-ui/src/progress/tests/index.spec.tsx
• git diff --check

Notes

Aligned with the React testing guidance to avoid tests that cannot fail meaningfully, avoid implementation-detail assertions, prefer observable contracts, and keep test code type-safe.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.33%. Comparing base (ee0068e) to head (1cd116a).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #38529      +/-   ##
==========================================
- Coverage   85.34%   85.33%   -0.01%     
==========================================
  Files        5012     5012              
  Lines      265775   265775              
  Branches    50519    50519              
==========================================
- Hits       226813   226812       -1     
- Misses      34512    34513       +1     
  Partials     4450     4450              

Flag	Coverage Δ
dify-ui	94.61% <ø> (-0.11%)	⬇️
web	84.89% <ø> (ø)

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