Add integration tests for gRPC BaseApiClient and utilities

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This PR adds comprehensive integration tests for the gRPC BaseApiClient and its utility functions (call_stub_method(), GrpcStreamBroadcaster) in a minimal and reproducible environment, including authentication and signing interceptors.

What's included

gRPC Service Definition

A minimal "hello world" style gRPC service with both unary and server-streaming methods:

service Greeter {
  rpc SayHello (HelloRequest) returns (HelloReply);
  rpc StreamHellos (HelloRequest) returns (stream HelloReply);
}

Test Infrastructure

• Test Server: Python gRPC server implementation using grpc.aio with authentication support
• Test Client: Client subclassing BaseApiClient with proper stub typing and generics
• Generated Stubs: Python gRPC classes generated from the proto definition

Integration Test Coverage

The tests validate core functionality in real client-server scenarios:

• Unary RPCs using the call_stub_method() utility function
• Server-streaming RPCs with proper async iteration patterns
• GrpcStreamBroadcaster functionality with multiple consumers and event handling
• Authentication interceptors (API key validation)
• Signing interceptors (HMAC signing with secrets)
• Timeout handling throughout all operations for reliability

CI-Friendly Design

• Mock-based tests for environments without gRPC dependencies
• Standalone validation tests to verify infrastructure setup
• Comprehensive documentation with usage examples

Example Usage

# Test client subclassing BaseApiClient
class GreeterClient(BaseApiClient[GreeterStub]):
    def __init__(self, server_url: str, *, auth_key: str | None = None):
        super().__init__(
            server_url=server_url,
            create_stub=lambda channel: GreeterStub(channel),
            auth_key=auth_key,
        )

# Integration test with authentication
async def test_authenticated_call():
    client = GreeterClient("grpc://localhost:50051", auth_key="test-key")
    request = HelloRequest(name="World")
    
    response = await call_stub_method(
        client.stub.SayHello,
        request,
        timeout=5.0,
    )
    
    assert "authenticated" in response.message

This ensures that BaseApiClient subclasses integrate generated stubs correctly, utility functions work in real usage scenarios, and security features (authentication/signing) function as intended.

Fixes #173.

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