fix(deps): cap protobuf below 7 (#1019)

## 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

Author: liujuanjuan1984

pyproject.toml

@@ -11,7 +11,7 @@ dependencies = [
   "httpx>=0.28.1",
   "httpx-sse>=0.4.0",
   "pydantic>=2.11.3",
-  "protobuf>=5.29.5",
+  "protobuf>=5.29.5,<7",
   "google-api-core>=1.26.0",
   "json-rpc>=1.15.0",
   "googleapis-common-protos>=1.70.0",