[Task]: Replace deprecated `FieldDescriptor.label` with `is_repeated` in `proto_utils`

[sokoliva]

Summary

src/a2a/utils/proto_utils.py currently uses the deprecated FieldDescriptor.label property to check whether a protobuf field is repeated. The non-deprecated alternative is FieldDescriptor.is_repeated, but adopting it requires a newer minimum version of protobuf than we currently declare, which would be a breaking change for downstream consumers.

Locations

All occurrences are in src/a2a/utils/proto_utils.py:

• src/a2a/utils/proto_utils.py:177 — _populate_message_from_params (or similar)
• src/a2a/utils/proto_utils.py:211 — _check_required_field_violation
• src/a2a/utils/proto_utils.py:252 — _recurse_validation
  Each site is marked with a TODO comment referencing this issue.

Proposed change

1. Bump the minimum required protobuf version in pyproject.toml to one that exposes FieldDescriptor.is_repeated as a stable, non-deprecated API.
2. Replace all field.label == FieldDescriptor.LABEL_REPEATED checks with field.is_repeated.
3. Replace all field.label != FieldDescriptor.LABEL_REPEATED checks with not field.is_repeated.
4. Remove the TODO comments and unused LABEL_REPEATED import if no longer needed.
5. Schedule the change as part of the next breaking release.

[liujuanjuan1984]

We hit this in downstream integration with a2a-sdk==1.0.2 on a real server path.

Concrete repro:

• a2a-sdk==1.0.2
• protobuf==7.34.1
• call path: DefaultRequestHandler.on_get_task() -> @validate_request_params -> validate_proto_required_fields() -> _check_required_field_violation()
• runtime error: AttributeError: 'google._upb._message.FieldDescriptor' object has no attribute 'label'

A few details from our side:

• a2a-sdk 1.0.2 currently declares Requires-Dist: protobuf>=5.29.5, so a normal resolver can select 7.x
• with protobuf 5.29.5 and 6.33.5, field.label is still present
• with protobuf 7.34.1, field.label is absent while field.is_repeated exists
• pinning downstream to protobuf<7 immediately resolves the crash

So the current field.label implementation is not compatible with all versions allowed by the published dependency range.

Given PR #1010 and the direction in this issue, I think there are two viable fixes:

1. short-term: tighten packaging to protobuf<7
2. follow-up: add a compatibility branch in proto_utils that prefers field.is_repeated when available and falls back to field.label

If useful, I can also open a PR for the packaging bound change.