test(core): stabilize ARM bazel remote-model and parallelism tests (#11330)

## Summary
- keep wiremock MockServer handles alive through async assertions in
remote model suite tests
- assert /models request count in remote_models_hide_picker_only_models
- use a slightly higher parallel timing threshold on aarch64 while
keeping existing x86 threshold

## Validation
- just fmt
- targeted tests:
- cargo test -p codex-core --test all
suite::remote_models::remote_models_merge_replaces_overlapping_model --
--exact
- cargo test -p codex-core --test all
suite::remote_models::remote_models_hide_picker_only_models -- --exact
- cargo test -p codex-core --test all
suite::tool_parallelism::shell_tools_run_in_parallel -- --exact
- soak loop: 40 iterations of all three targeted tests

## Notes
- cargo test -p codex-core has one unrelated local-env failure in
shell_snapshot::tests::try_new_creates_and_deletes_snapshot_file from
exported certificate env content in this workspace.
- local bazel test //codex-rs/core:core-all-test failed to build due
missing rust-objcopy in this host toolchain.

codex-rs/core/tests/suite/rmcp_client.rs

@@ -247,6 +247,31 @@ async fn stdio_image_responses_round_trip() -> anyhow::Result<()> {
         .await?;
     let session_model = fixture.session_configured.model.clone();
 
+    let tools_ready_deadline = Instant::now() + Duration::from_secs(30);
+    loop {
+        fixture.codex.submit(Op::ListMcpTools).await?;
+        let list_event = core_test_support::wait_for_event_with_timeout(
+            &fixture.codex,
+            |ev| matches!(ev, EventMsg::McpListToolsResponse(_)),
+            Duration::from_secs(10),
+        )
+        .await;
+        let EventMsg::McpListToolsResponse(tool_list) = list_event else {
+            unreachable!("event guard guarantees McpListToolsResponse");
+        };
+        if tool_list.tools.contains_key(&tool_name) {
+            break;
+        }
+
+        let available_tools: Vec<&str> = tool_list.tools.keys().map(String::as_str).collect();
+        if Instant::now() >= tools_ready_deadline {
+            panic!(
+                "timed out waiting for MCP tool {tool_name} to become available; discovered tools: {available_tools:?}"
+            );
+        }
+        sleep(Duration::from_millis(200)).await;
+    }
+
     fixture
         .codex
         .submit(Op::UserTurn {