fix(utils): protobuf-compatible repeated field detection

[piyushbag]

Summary

• Add _is_field_repeated() in proto_utils that prefers field.is_repeated and falls back to field.label == FieldDescriptor.LABEL_REPEATED when the newer API is unavailable.
• Replace the three repeated-field checks in REST param parsing and proto required-field validation with the helper.
• Add unit tests for scalar/repeated fields and for the label fallback path.

Problem

proto_utils used the deprecated FieldDescriptor.label API. Protobuf 7 removes label, which breaks validation on newer runtimes even though the SDK dependency range can resolve protobuf 7.x in some environments.

Test plan

• uv run pytest tests/utils/test_proto_utils.py -q
• Verified _is_field_repeated against protobuf 6.x locally (is_repeated + label both present)

Fixes #1011

[github-actions]

🧪 Code Coverage (vs main)

⬇️ Download Full Report

	Base	PR	Delta
src/a2a/utils/proto_utils.py	90.87%	91.08%	🟢 +0.21%
Total	92.97%	92.98%	⚪️ 0.00%

Generated by coverage-comment.yml

[gemini-code-assist]

Code Review

This pull request introduces a helper function _is_field_repeated to safely check if a protobuf field is repeated across different protobuf versions, replacing deprecated field.label checks. It also adds unit tests for this helper. The review feedback points out that monkeypatching FieldDescriptor in test_fallback_when_is_repeated_missing will fail in environments using the fast C++ implementation of Protobuf because C-extension types are read-only. A code suggestion is provided to use a simple mock object instead.

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[gemini-code-assist]

Modifying the attributes or properties of FieldDescriptor (or its type) using monkeypatch will fail with a TypeError in environments where the fast C++ implementation of Protobuf is used (which is the default in many environments, as FieldDescriptor is a C-extension type and its attributes/type are read-only).

Instead of monkeypatching the built-in FieldDescriptor class, you can use a simple mock object or a custom class that lacks the is_repeated attribute to safely test the fallback path.

Suggested change

	def test_fallback_when_is_repeated_missing(self, monkeypatch):
	field = AgentSkill.DESCRIPTOR.fields_by_name['tags']
	def raise_attribute_error(_self):
	raise AttributeError('is_repeated')
	monkeypatch.setattr(
	type(field),
	'is_repeated',
	property(raise_attribute_error),
	)
	assert proto_utils._is_field_repeated(field) is True
	def test_fallback_when_is_repeated_missing(self):
	class MockField:
	label = proto_utils.FieldDescriptor.LABEL_REPEATED
	field = MockField()
	assert proto_utils._is_field_repeated(field) is True

References

1. When checking for repeated fields in Protobuf, access descriptor constants via the FieldDescriptor class (e.g., FieldDescriptor.LABEL_REPEATED) rather than directly from the field instance (e.g., field.is_repeated).