Use MCP bindings as the step tool catalog (#36360)

## What changed

- Read the frozen MCP tool catalog directly from the step-scoped `McpBinding`
  when building tool routers, plugin injections, and connector state.
- Remove the redundant `Vec<ToolInfo>` from `StepContext` and stop returning a
  duplicate catalog from `built_tools`.

GitOrigin-RevId: d12d694e1224da3f6ccb1960868491f0d80c6e19

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

@@ -7,7 +7,6 @@ use crate::tools::router::ToolRouter;
 use codex_exec_server::ExecutorCapabilityDiscoverySnapshot;
 use codex_exec_server::ResolvedSelectedCapabilityRoot;
 use codex_mcp::McpBinding;
-use codex_mcp::ToolInfo;
 
 /// Request-scoped state that may change between model sampling requests.
 pub(crate) struct StepContext {
@@ -19,8 +18,6 @@ pub(crate) struct StepContext {
     pub(crate) executor_capability_discovery: Option<Arc<ExecutorCapabilityDiscoverySnapshot>>,
     /// The exact MCP connections, configuration, and catalog captured for this step.
     pub(crate) mcp: Arc<McpBinding>,
-    /// The fixed MCP tool list used for this exact sampling request.
-    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.