Complete the A2A HTTP transport implementation for server mode:

- Update TransportSelector to return A2aRemoteTransport when server URL
  is configured (previously returned stub A2aHttpTransport)
- Add A2aTransport type alias to __init__.py exports
- Add BDD feature file for A2aRemoteTransport with 17 coverage scenarios
- Add step definitions for A2aRemoteTransport tests covering:
  - connect/disconnect/is_connected lifecycle
  - send() raises NotImplementedError (requires async context)
  - send_async() with mocked aiohttp session
  - Bearer token authentication headers
  - HTTP error handling (500 status, ClientError)
  - TransportSelector integration tests

ISSUES CLOSED: #692

@@ -0,0 +127,4 @@
+@when("I try to connect the remote transport with non-string base_url")
+def step_connect_non_string_url(context: Any) -> None:
+    try:
+        context.transport.connect(None)  # type: ignore[arg-type]

@@ -20,0 +150,4 @@
+            # Create session if needed, with timeout and TLS configuration
+            if self._session is None:
+                timeout = aiohttp.ClientTimeout(total=_DEFAULT_TIMEOUT_SECONDS)
+                ssl_context: ssl.SSLContext | bool = bool(self._tls_verify)

@@ -0,0 +150,4 @@
+
+
+@when("I try to send a request via the disconnected remote transport")
+def step_send_disconnected_remote(context: Any) -> None: