fix: update proto2pydantic to v0.5.0 with ruff-compliant output

- gen_proto.sh: bump proto2pydantic v0.4.0 -> v0.5.0
- a2a_pydantic.py: improved millisecond formatting in timestamp serializer

Author: brucearctor

scripts/gen_proto.sh

@@ -2,8 +2,8 @@
 set -e
 
 # Install proto2pydantic plugin for Pydantic model generation
-echo "Installing protoc-gen-proto2pydantic@v0.4.0..."
-go install github.com/protocgen/proto2pydantic@v0.4.0
+echo "Installing protoc-gen-proto2pydantic@v0.5.0..."
+go install github.com/protocgen/proto2pydantic@v0.5.0
 
 # Run buf generate to regenerate protobuf code and OpenAPI spec
 npx --yes @bufbuild/buf generate

src/a2a/types/a2a_pydantic.py

@@ -128,7 +128,7 @@ def _serialize_timestamp(cls, v: datetime | None, _info: Any) -> str | None:
         """Serialize datetime to RFC 3339 with UTC 'Z' suffix for ProtoJSON."""
         if v is None:
             return None
-        return v.strftime('%Y-%m-%dT%H:%M:%S.') + f'{v.microsecond:06d}'[:3] + 'Z'
+        return v.strftime('%Y-%m-%dT%H:%M:%S.') + f'{v.microsecond // 1000:03d}' + 'Z'
 
 
 
@@ -528,7 +528,7 @@ def _serialize_timestamp(cls, v: datetime | None, _info: Any) -> str | None:
         """Serialize datetime to RFC 3339 with UTC 'Z' suffix for ProtoJSON."""
         if v is None:
             return None
-        return v.strftime('%Y-%m-%dT%H:%M:%S.') + f'{v.microsecond:06d}'[:3] + 'Z'
+        return v.strftime('%Y-%m-%dT%H:%M:%S.') + f'{v.microsecond // 1000:03d}' + 'Z'