Negotiate MCP extensions per app-server session (#36910)

## Why

App-server clients need to advertise structured MCP extension settings, including supported MCP App UI MIME types, rather than only opting into OpenAI form elicitation with a boolean.

## What changed

- Add an `extensions` map to initialize capabilities and preserve `mcpServerOpenaiFormElicitation` as a legacy alias for `openai/form`.
- Capture the declared extension profile when a thread is started, resumed, or forked, propagate it to subagents, and advertise it to downstream MCP servers during initialization.
- Keep the profile stable for the lifetime of the loaded session instead of changing it on later turns or direct tool calls.

## Testing

- Cover extension conversion, downstream MCP initialization, session isolation, legacy form support, and subagent inheritance.

GitOrigin-RevId: fbcedbb74ce788e574b0f884a4c45c4cedb9de54

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

@@ -1,5 +1,6 @@
 pub use binding::McpBinding;
 pub use binding::PreparedMcpCall;
+pub use client_capabilities::client_mcp_extensions;
 pub use codex_rmcp_client::McpProtocolMode;
 pub use connection_manager::tool_is_model_visible;
 pub use elicitation::ElicitationLifecycle;
@@ -86,6 +87,7 @@ pub(crate) mod auth_elicitation;
 mod binding;
 pub(crate) mod binding_clients;
 mod catalog;
+mod client_capabilities;
 pub(crate) mod codex_apps;
 pub(crate) mod connection_manager;
 pub(crate) mod elicitation;