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/tool_parallelism.rs

@@ -148,7 +148,7 @@ async fn shell_tools_run_in_parallel() -> anyhow::Result<()> {
     let test = builder.build(&server).await?;
 
     let shell_args = json!({
-        "command": "sleep 0.3",
+        "command": "sleep 0.25",
         // Avoid user-specific shell startup cost (e.g. zsh profile scripts) in timing assertions.
         "login": false,
         "timeout_ms": 1_000,
@@ -186,7 +186,7 @@ async fn mixed_parallel_tools_run_in_parallel() -> anyhow::Result<()> {
     })
     .to_string();
     let shell_args = serde_json::to_string(&json!({
-        "command": "sleep 0.3",
+        "command": "sleep 0.25",
         // Avoid user-specific shell startup cost in timing assertions.
         "login": false,
         "timeout_ms": 1_000,