Preserve user input when MCP startup is interrupted (#34839)

## Why

Interrupting a turn while MCP tools were still starting could abort before the
submitted user input was recorded in conversation history.

## What changed

- Build and retain the MCP tool list and tool router as part of each step
  snapshot, using the turn cancellation token during tool discovery.
- Record turn hooks and submitted input when cancellation occurs during startup
  prewarming, pre-sampling compaction, or first-step capture.
- Reuse the captured router for sampling, compaction, and prompt debugging so
  advertised and executable tools share the same request-scoped view.

## Testing

Add coverage that interrupts each affected MCP startup phase and verifies that
the user prompt remains in history before the turn-aborted marker.

GitOrigin-RevId: 6e96c6814357a55545d69b580e28f35933cfadf7

codex-rs/core/src/session/step_context.rs

@@ -4,13 +4,12 @@ use crate::agents_md::LoadedAgentsMd;
 use crate::environment_selection::TurnEnvironmentSnapshot;
 use crate::session::McpRuntimeSnapshot;
 use crate::session::turn_context::TurnContext;
+use crate::tools::router::ToolRouter;
 use codex_exec_server::ExecutorCapabilityDiscoverySnapshot;
 use codex_exec_server::ResolvedSelectedCapabilityRoot;
 use codex_mcp::ToolInfo;
-use tokio::sync::OnceCell;
 
 /// Request-scoped state that may change between model sampling requests.
-#[derive(Debug)]
 pub(crate) struct StepContext {
     pub(crate) turn: Arc<TurnContext>,
     pub(crate) environments: TurnEnvironmentSnapshot,
@@ -21,34 +20,9 @@ pub(crate) struct StepContext {
     /// The exact MCP config and manager used to advertise and execute tools for this step.
     pub(crate) mcp: Arc<McpRuntimeSnapshot>,
     /// The fixed MCP tool list used for this exact sampling request.
-    mcp_tool_snapshot: OnceCell<Vec<ToolInfo>>,
+    pub(crate) mcp_tools: Vec<ToolInfo>,
+    /// The finalized tool plan advertised and executed for this exact sampling request.
+    pub(crate) tool_router: Arc<ToolRouter>,
     /// The canonical AGENTS.md value observed with this environment snapshot.
     pub(crate) loaded_agents_md: Option<Arc<LoadedAgentsMd>>,
 }
-
-impl StepContext {
-    pub(crate) fn new(
-        turn: Arc<TurnContext>,
-        environments: TurnEnvironmentSnapshot,
-        selected_capability_roots: Vec<ResolvedSelectedCapabilityRoot>,
-        executor_capability_discovery: Option<Arc<ExecutorCapabilityDiscoverySnapshot>>,
-        mcp: Arc<McpRuntimeSnapshot>,
-        loaded_agents_md: Option<Arc<LoadedAgentsMd>>,
-    ) -> Self {
-        Self {
-            turn,
-            environments,
-            selected_capability_roots,
-            executor_capability_discovery,
-            mcp,
-            mcp_tool_snapshot: OnceCell::new(),
-            loaded_agents_md,
-        }
-    }
-
-    pub(crate) async fn mcp_tools(&self) -> &[ToolInfo] {
-        self.mcp_tool_snapshot
-            .get_or_init(|| self.mcp.manager().list_all_tools())
-            .await
-    }
-}