test: use assertions instead of conditional guards in resumption test

The session ID and protocol version are always set after initialization,
so assert their presence rather than silently skipping the headers.

Author: maxisbey

tests/shared/test_streamable_http.py

@@ -1312,10 +1312,10 @@ async def run_tool():
 
     # Now resume the session with the same mcp-session-id and protocol version
     headers: dict[str, Any] = {}
-    if captured_session_id:
-        headers[MCP_SESSION_ID_HEADER] = captured_session_id
-    if captured_protocol_version:
-        headers[MCP_PROTOCOL_VERSION_HEADER] = captured_protocol_version
+    assert captured_session_id is not None
+    headers[MCP_SESSION_ID_HEADER] = captured_session_id
+    assert captured_protocol_version is not None
+    headers[MCP_PROTOCOL_VERSION_HEADER] = captured_protocol_version
 
     async with create_mcp_http_client(headers=headers) as httpx_client:
         async with streamable_http_client(f"{server_url}/mcp", http_client=httpx_client) as (