fix(server): preserve required empty agent card skills

[Ha1baraA11]

Description

Thank you for opening a Pull Request!
Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Follow the CONTRIBUTING Guide.
• Make your Pull Request title in the https://www.conventionalcommits.org/ specification.
  • Important Prefixes for release-please:
    • fix: which represents bug fixes, and correlates to a SemVer patch.
    • feat: represents new features, and correlates to a SemVer minor.
    • feat!:, or fix!:, refactor!:, etc., which are breaking changes (indicated by the !) and will result in a SemVer major.
• Ensure the tests and linter pass (Run bash scripts/format.sh from the repository root to format)
• Appropriate docs were updated (not needed for this serialization fix)

Summary

• Preserve the required skills field as an empty list when serializing an AgentCard with no skills.
• Keep protobuf's omission behavior for other optional empty repeated fields.

Test plan

• ./scripts/lint.sh (ruff check, ruff format, ty check)
• uv run pytest -k 'not cross_version' (1987 passed, 92 skipped, 3 xfailed, 1 xpassed)
• uv run pytest -k 'not cross_version' --cov=src --cov-report=term-missing (93% coverage)
• Manual route and response-helper regression tests (21 passed)

Fixes #1261 🦕

[github-actions]

🧪 Code Coverage (vs main)

⬇️ Download Full Report

	Base	PR	Delta
src/a2a/server/request_handlers/response_helpers.py	95.71%	95.77%	🟢 +0.06%
Total	93.06%	93.06%	⚪️ 0.00%

Generated by coverage-comment.yml

[mykytanetipa]

The AgentCard usecase reported in the #1261 is not spec compliant, closing this PR.