Keep MCP tool calls bound to their thread (#36355)

## Why

Threads can configure the same MCP server name with different runtimes. Tool calls
must use the runtime associated with the thread that issued the call.

## What changed

- Route MCP readiness checks and call preparation through the invoking session.
- Add regression coverage that alternates calls between two threads and verifies
  that each thread continues using its own MCP server process and configuration.

GitOrigin-RevId: 5735f7b29a579f545351385ab01a575e35ead01e

codex-rs/core/src/mcp_tool_call.rs

@@ -140,16 +140,7 @@ pub(crate) async fn handle_mcp_tool_call(
         arguments: arguments_value.clone(),
     };
 
-    sess.refresh_mcp_if_dirty().await;
-    let current_binding = sess
-        .services
-        .mcp_runtime
-        .current_binding_for_call(&server)
-        .await;
-    let Some(prepared_call) = current_binding
-        .as_ref()
-        .and_then(|binding| binding.prepare_call(&server, &tool_name))
-    else {
+    let Some(prepared_call) = sess.prepare_mcp_call(&server, &tool_name).await else {
         let item_metadata =
             McpToolCallItemMetadata::from_tool_metadata(&server, /*metadata*/ None);
         let result = notify_mcp_tool_call_skip(