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/core/src/session/step_context.rs

@@ -6,7 +6,6 @@ use crate::session::McpRuntimeSnapshot;
 use crate::session::turn_context::TurnContext;
 use codex_exec_server::ExecutorCapabilityDiscoverySnapshot;
 use codex_exec_server::ResolvedSelectedCapabilityRoot;
-use codex_extension_api::ExtensionData;
 use codex_mcp::ToolInfo;
 use tokio::sync::OnceCell;
 
@@ -23,8 +22,6 @@ pub(crate) struct StepContext {
     pub(crate) mcp: Arc<McpRuntimeSnapshot>,
     /// The fixed MCP tool list used for this exact sampling request.
     mcp_tool_snapshot: OnceCell<Vec<ToolInfo>>,
-    /// Extension capabilities bound to this exact sampling step.
-    pub(crate) extension_data: ExtensionData,
     /// The canonical AGENTS.md value observed with this environment snapshot.
     pub(crate) loaded_agents_md: Option<Arc<LoadedAgentsMd>>,
 }
@@ -38,7 +35,6 @@ impl StepContext {
         mcp: Arc<McpRuntimeSnapshot>,
         loaded_agents_md: Option<Arc<LoadedAgentsMd>>,
     ) -> Self {
-        let extension_data = ExtensionData::new(turn.sub_id.clone());
         Self {
             turn,
             environments,
@@ -46,7 +42,6 @@ impl StepContext {
             executor_capability_discovery,
             mcp,
             mcp_tool_snapshot: OnceCell::new(),
-            extension_data,
             loaded_agents_md,
         }
     }