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/tools/handlers/multi_agents_tests.rs

@@ -30,6 +30,7 @@ use codex_protocol::ThreadId;
 use codex_protocol::config_types::ApprovalsReviewer;
 use codex_protocol::config_types::ServiceTier;
 use codex_protocol::config_types::ShellEnvironmentPolicy;
+use codex_protocol::mcp::ClientMcpExtensions;
 use codex_protocol::models::BaseInstructions;
 use codex_protocol::models::ContentItem;
 use codex_protocol::models::FunctionCallOutputBody;
@@ -2730,7 +2731,7 @@ async fn resume_agent_restores_closed_agent_and_accepts_send_input() {
             })]),
             AuthManager::from_auth_for_testing(CodexAuth::from_api_key("dummy")),
             /*parent_trace*/ None,
-            /*supports_openai_form_elicitation*/ false,
+            ClientMcpExtensions::default(),
         )
         .await
         .expect("start thread");