Attribute OpenAI docs MCP requests to Codex (#33424)

## What changed

- Add `source=codex` to requests sent to the OpenAI developer docs MCP endpoint.
- Preserve OAuth discovery URLs and leave clients for other MCP servers unchanged.

## Testing

- Add unit coverage for regular and streaming HTTP requests, OAuth discovery, and non-docs MCP clients.

GitOrigin-RevId: d04e19fe6fc2db0b5cd7bfb429f0cb821c49b28a

codex-rs/codex-mcp/src/lib.rs

@@ -86,6 +86,7 @@ pub(crate) mod codex_apps;
 pub(crate) mod connection_manager;
 pub(crate) mod elicitation;
 pub(crate) mod mcp;
+mod openai_docs_source_attribution;
 mod plugin_config;
 mod resource_client;
 pub(crate) mod rmcp_client;