Add ValueError for missing transport key: sessions.py (#198)

This pull request adds a `ValueError` to validate missing `transport`
keys in `MultiServerMCPClient` server configurations. The error message
includes valid `transport` values (`stdio`, `sse`, `websocket`,
`streamable_http`) and a link to the documentation for clarity.

The change was prompted by Grok and ChatGPT suggesting an invalid
configuration, such as:


```
{
  "mcpServers": {
    "gitmcp": {
      "command": "npx",
      "args": ["mcp-remote", "https://gitmcp.io/{owner}/{repo}"]
    }
  }
}
```

This caused a `KeyError: transport` due to incorrect nesting and missing
`transport`. A warning was added to guide users to the correct top-level
format, improving error clarity and preventing similar issues.

---------

Co-authored-by: Eugene Yurtsev <[email protected]>

Author: ozanayrikan

langchain_mcp_adapters/sessions.py

@@ -275,6 +275,15 @@ async def create_session(
     Yields:
         A ClientSession
     """
+
+    if "transport" not in connection:
+        raise ValueError(
+            "Configuration error: Missing 'transport' key in server configuration. "
+            "Each server must include 'transport' with one of: "
+            "'stdio', 'sse', 'websocket', 'streamable_http'. "
+            "Please refer to the langchain-mcp-adapters documentation for more details."
+        )
+
     transport = connection["transport"]
     if transport == "sse":
         if "url" not in connection:
@@ -329,5 +338,6 @@ async def create_session(
             yield session
     else:
         raise ValueError(
-            f"Unsupported transport: {transport}. Must be one of: 'stdio', 'sse', 'websocket', 'streamable_http'"
+            f"Unsupported transport: {transport}. "
+            f"Must be one of: 'stdio', 'sse', 'websocket', 'streamable_http'"
         )