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/core/src/session/mcp.rs

@@ -558,26 +558,6 @@ impl Session {
             .await
     }
 
-    pub(crate) async fn set_openai_form_elicitation_support(
-        &self,
-        supported: bool,
-    ) -> anyhow::Result<()> {
-        if self
-            .services
-            .supports_openai_form_elicitation
-            .load(std::sync::atomic::Ordering::Relaxed)
-            == supported
-        {
-            return Ok(());
-        }
-
-        self.services
-            .supports_openai_form_elicitation
-            .store(supported, std::sync::atomic::Ordering::Relaxed);
-        self.request_mcp_runtime_refresh();
-        Ok(())
-    }
-
     pub(crate) async fn refresh_mcp_servers_now(
         &self,
         turn_context: &TurnContext,