Use step environments for MCP file uploads (#35878)

## Why

An MCP tool call can run after its selected environment becomes ready, while the
turn context still contains the earlier starting-state snapshot. File argument
rewriting must use the environment available to the current step so it can
resolve and upload local files in that case.

## What changed

- Pass `StepContext` through MCP file argument rewriting.
- Resolve the primary environment from the step snapshot while retaining
  turn-scoped configuration and metadata.
- Add coverage for an environment that transitions from starting to ready
  during a turn.

GitOrigin-RevId: 935bea18ee425aabeacf4505eaff90565f9751e9

codex-rs/core/src/mcp_tool_call.rs

@@ -428,7 +428,7 @@ async fn handle_approved_mcp_tool_call(
                         .await;
                     let rewritten_arguments = rewrite_mcp_tool_arguments_for_openai_files(
                         sess,
-                        turn_context,
+                        step_context,
                         arguments_value,
                         metadata.openai_file_input_optional_fields.as_ref(),
                     )