### What happened?

When using LiteLLM Proxy as an **MCP gateway**, `CustomLogger.async_post_mcp_tool_call_hook` is typed to return `Optional[MCPPostCallResponseObject]`, and the logging wrapper explicitly parses and returns the (possibly modified) response. However, **every caller of the hook discards this return value**, so returning a modified `MCPPostCallResponseObject` from a custom logger has **no effect** on the response delivered to the MCP client.

### Why this is confusing

The public API strongly signals that returning a modified response is the intended way to rewrite a tool result:

- The hook signature returns `Optional[MCPPostCallResponseObject]`. - `MCPPostCallResponseObject.mcp_tool_call_response` holds the tool output content. - `LiteLLMLoggingObj.async_post_mcp_tool_call_hook` collects the callback's return value, runs `_parse_post_mcp_call_hook_response()`, and returns the unpacked `mcp_tool_call_response`, with the inline comment:

``` # if any of the callbacks modify the response, use the modified response # current implementation returns the first modified response ```

But the two call sites throw that return value away:

- `litellm/proxy/_experiment…