fix(deps): cap protobuf below 7

[liujuanjuan1984]

Summary

• tighten the published protobuf dependency range to <7
• keep the declared range aligned with the current proto_utils implementation, which still relies on FieldDescriptor.label
• update uv.lock metadata to match

Why

a2a-sdk 1.0.2 currently publishes protobuf>=5.29.5, which allows resolvers to select protobuf 7.x.

In downstream testing, a2a-sdk==1.0.2 with protobuf==7.34.1 crashes on the DefaultRequestHandler.on_get_task() path:

• @validate_request_params
• validate_proto_required_fields()
• _check_required_field_violation()
• AttributeError: 'google._upb._message.FieldDescriptor' object has no attribute 'label'

This PR applies the minimal packaging fix that matches the current implementation choice from #1010 while broader code-level compatibility is discussed in #1011.

Relates to #1011

Validation

• uv run ruff check --fix
• uv run ruff format
• uv run mypy src
• uv run pyright src
• uv run pytest

pytest result locally:

• 1671 passed, 90 skipped, 3 xfailed, 4 errors
• the 4 errors were all in tests/integration/cross_version/client_server/test_client_server.py
• they were startup failures for the server_0_3.py subprocess path using uv run --with a2a-sdk[grpc]==0.3.24 --no-project ...
• these failures do not exercise the changed dependency declaration in this PR

[github-actions]

🧪 Code Coverage (vs main)

⬇️ Download Full Report

	Base	PR	Delta
src/a2a/server/events/event_queue_v2.py	91.71%	91.19%	🔴 -0.52%
src/a2a/utils/telemetry.py	91.41%	90.63%	🔴 -0.78%
Total	93.03%	93.00%	🔴 -0.02%

Generated by coverage-comment.yml

[gemini-code-assist]

Code Review

This pull request updates the protobuf dependency version constraint to >=5.29.5,<7 in both pyproject.toml and uv.lock. I have no feedback to provide.

[sokoliva]

Hi @liujuanjuan1984,

Thank you for bringing this to our attention and for fixing the issue. I replicated the error you got and confirmed the fix you offered.