fix: encode extended download filenames

[cxymds]

Description

Correct RFC 8187 encoding for extended download filenames. encodeURIComponent leaves ', (, ) and * unescaped, but those characters are not valid in the filename* value. Encode them so Unicode object names containing these characters are preserved by browsers instead of falling back to the sanitized ASCII filename.

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactoring
• Performance improvement
• Test improvements
• Security fix

Testing

• Unit tests added/updated
• Manual testing completed

git diff --check
pnpm type-check
pnpm lint
pnpm format:check
pnpm test:run

All checks pass; the test suite reports 600 passing tests.

Checklist

• Code follows the project's style guidelines
• Self-review completed
• TypeScript types are properly defined
• All commit messages are in English (Conventional Commits)
• All existing tests pass
• No new dependencies added, or they are justified

Related Issues

Related to rustfs/rustfs#8027 and #238.

Screenshots (if applicable)

N/A: this change affects HTTP header encoding only; no visual UI is introduced.

Additional Notes

This is a follow-up to #238 and is intentionally limited to the RFC 8187 filename encoding boundary.