fix: Add validation for transport types in ClientFactory (#396)

### Description

This update introduces upfront validation for transport protocols within
the `ClientFactory`. Previously, if an invalid or misspelled transport
type was provided in the `ClientConfig`, it would be silently ignored.
This could lead to confusing `ValueError('no compatible transports
found.')` errors later on, making it difficult to debug
misconfigurations.

### Changes

* **`src/a2a/client/client_factory.py`**: Added a validation loop at the
beginning of the `create` method to check all transport types provided
in the `ClientConfig`.
* **`tests/client/test_client_factory.py`**: Added a new unit test,
`test_client_factory_invalid_transport_in_config`, to confirm that a
`ValueError` is raised when an invalid transport string is used.

Author: pankaj-bind

src/a2a/client/client_factory.py

@@ -91,7 +91,7 @@ def _register_defaults(
             if GrpcTransport is None:
                 raise ImportError(
                     'To use GrpcClient, its dependencies must be installed. '
-                    'You can install them with \'pip install "a2a-sdk[grpc]"\''
+                    'You can install them with \'pip install "a2a-sdk[grpc]"\'',
                 )
             self.register(
                 TransportProtocol.grpc,
@@ -124,6 +124,20 @@ def create(
           If there is no valid matching of the client configuration with the
           server configuration, a `ValueError` is raised.
         """
+        valid_transports = {member.value for member in TransportProtocol}
+        configured_transports = set(self._config.supported_transports)
+
+        invalid_transports = configured_transports.difference(valid_transports)
+        if invalid_transports:
+            invalid_str = ', '.join(
+                sorted(f"'{t}'" for t in invalid_transports)
+            )
+            valid_str = ', '.join(sorted(valid_transports))
+            raise ValueError(
+                f'Unsupported transport type(s) in ClientConfig: {invalid_str}. '
+                f'Valid types are: {valid_str}'
+            )
+
         server_preferred = card.preferred_transport or TransportProtocol.jsonrpc
         server_set = {server_preferred: card.url}
         if card.additional_interfaces:

tests/client/test_client_factory.py

@@ -103,3 +103,32 @@ def test_client_factory_no_compatible_transport(base_agent_card: AgentCard):
     factory = ClientFactory(config)
     with pytest.raises(ValueError, match='no compatible transports found'):
         factory.create(base_agent_card)
+
+
+@pytest.mark.parametrize(
+    ('invalid_transports', 'expected_match'),
+    [
+        (
+            ['invalid-transport'],
+            "Unsupported transport type\\(s\\) in ClientConfig: 'invalid-transport'",
+        ),
+        (
+            ['invalid-1', 'another-bad-one'],
+            "Unsupported transport type\\(s\\) in ClientConfig: 'another-bad-one', 'invalid-1'",
+        ),
+    ],
+)
+def test_client_factory_invalid_transport_in_config(
+    base_agent_card: AgentCard, invalid_transports, expected_match
+):
+    """Verify that the factory raises an error for unknown transport types."""
+    config = ClientConfig(
+        httpx_client=httpx.AsyncClient(),
+        supported_transports=[
+            TransportProtocol.jsonrpc,
+            *invalid_transports,
+        ],
+    )
+    factory = ClientFactory(config)
+    with pytest.raises(ValueError, match=expected_match):
+        factory.create(base_agent_card)