Remove step-scoped data from extension contributors (#34734)

## What changed

- Remove the step-scoped `ExtensionData` argument from context, turn-input, and tool contributors.
- Pass the host's optional `McpResourceClient` through `ThreadStartInput` so extensions can retain session capabilities explicitly.
- Keep the MCP resource client in skills-owned session state for catalog loading and skill tools.

GitOrigin-RevId: bafa77bcd998aff408d6a396c5fd9ac268c4cce4

codex-rs/ext/extension-api/src/contributors/context.rs

@@ -15,8 +15,6 @@ pub struct TurnContextContributionInput<'a> {
     pub thread_store: &'a ExtensionData,
     /// Store scoped to this turn.
     pub turn_store: &'a ExtensionData,
-    /// Store scoped to this sampling step.
-    pub step_store: &'a ExtensionData,
     /// Effective model context window for this turn, when known.
     pub model_context_window: Option<i64>,
 }

codex-rs/ext/extension-api/src/contributors/thread_lifecycle.rs

@@ -1,4 +1,7 @@
+use std::sync::Arc;
+
 use crate::ExtensionData;
+use codex_mcp::McpResourceClient;
 use codex_protocol::protocol::SessionSource;
 use codex_protocol::protocol::TurnEnvironmentSelection;
 
@@ -20,6 +23,8 @@ pub struct ThreadStartInput<'a, C> {
     pub persistent_thread_state_available: bool,
     /// Execution environments selected for this thread.
     pub environments: &'a [TurnEnvironmentSelection],
+    /// MCP resource access supplied by the host for this session.
+    pub mcp_resource_client: Option<Arc<McpResourceClient>>,
     /// Store scoped to the host session runtime.
     pub session_store: &'a ExtensionData,
     /// Store scoped to this thread runtime.

codex-rs/ext/extension-api/src/contributors/world_state.rs

@@ -20,8 +20,6 @@ pub struct WorldStateContributionInput<'a> {
     pub session_store: &'a ExtensionData,
     pub thread_store: &'a ExtensionData,
     pub turn_store: &'a ExtensionData,
-    /// Host capabilities bound to this exact sampling step.
-    pub step_store: &'a ExtensionData,
 }
 
 /// What the harness knows about the previous value of one extension-owned section.