fix: changes to test that should help them pass on Windows under Bazel (#16662)

https://github.com/openai/codex/pull/16460 was a large PR created by
Codex to try to get the tests to pass under Bazel on Windows. Indeed, it
successfully ran all of the tests under `//codex-rs/core:` with its
changes to `codex-rs/core/`, though the full set of changes seems to be
too broad.

This PR tries to port the key changes, which are:

- Under Bazel, the `USERNAME` environment variable is not guaranteed to
be set on Windows, so for tests that need a non-empty env var as a
convenient substitute for an env var containing an API key, just use
`PATH`. Note that `PATH` is unlikely to contain characters that are not
allowed in an HTTP header value.
- Specify `"powershell.exe"` instead of just `"powershell"` in case the
`PATHEXT` env var gets lost in the shuffle.

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

@@ -196,7 +196,7 @@ $lines | Select-Object -Skip 1 | Set-Content -Path tokens.txt
 "#,
             )?;
             (
-                "powershell".to_string(),
+                "powershell.exe".to_string(),
                 vec![
                     "-NoProfile".to_string(),
                     "-ExecutionPolicy".to_string(),
@@ -2590,10 +2590,18 @@ async fn incomplete_response_emits_content_filter_error_message() -> anyhow::Res
     Ok(())
 }
 
+/// We try to avoid setting env vars in tests because std::env::set_var() is
+/// process-wide and unsafe. Though for this test, we want to simulate the
+/// presence of an environment variable that the provider will read for auth, so
+/// we pick a commonly existing env var that is guaranteed to have a non-empty
+/// value on both Windows and Unix. Note that this test must also work when run
+/// under Bazel in CI, which uses a restricted environment, so PATH seems like
+/// the safest choice.
+const EXISTING_ENV_VAR_WITH_NON_EMPTY_VALUE: &str = "PATH";
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn azure_overrides_assign_properties_used_for_responses_url() {
     skip_if_no_network!();
-    let existing_env_var_with_random_value = if cfg!(windows) { "USERNAME" } else { "USER" };
 
     // Mock server
     let server = MockServer::start().await;
@@ -2611,11 +2619,11 @@ async fn azure_overrides_assign_properties_used_for_responses_url() {
         .and(path("/openai/responses"))
         .and(query_param("api-version", "2025-04-01-preview"))
         .and(header_regex("Custom-Header", "Value"))
-        .and(header_regex(
+        .and(header(
             "Authorization",
             format!(
                 "Bearer {}",
-                std::env::var(existing_env_var_with_random_value).unwrap()
+                std::env::var(EXISTING_ENV_VAR_WITH_NON_EMPTY_VALUE).unwrap()
             )
             .as_str(),
         ))
@@ -2628,7 +2636,7 @@ async fn azure_overrides_assign_properties_used_for_responses_url() {
         name: "custom".to_string(),
         base_url: Some(format!("{}/openai", server.uri())),
         // Reuse the existing environment variable to avoid using unsafe code
-        env_key: Some(existing_env_var_with_random_value.to_string()),
+        env_key: Some(EXISTING_ENV_VAR_WITH_NON_EMPTY_VALUE.to_string()),
         experimental_bearer_token: None,
         auth: None,
         query_params: Some(std::collections::HashMap::from([(
@@ -2679,7 +2687,6 @@ async fn azure_overrides_assign_properties_used_for_responses_url() {
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn env_var_overrides_loaded_auth() {
     skip_if_no_network!();
-    let existing_env_var_with_random_value = if cfg!(windows) { "USERNAME" } else { "USER" };
 
     // Mock server
     let server = MockServer::start().await;
@@ -2697,11 +2704,11 @@ async fn env_var_overrides_loaded_auth() {
         .and(path("/openai/responses"))
         .and(query_param("api-version", "2025-04-01-preview"))
         .and(header_regex("Custom-Header", "Value"))
-        .and(header_regex(
+        .and(header(
             "Authorization",
             format!(
                 "Bearer {}",
-                std::env::var(existing_env_var_with_random_value).unwrap()
+                std::env::var(EXISTING_ENV_VAR_WITH_NON_EMPTY_VALUE).unwrap()
             )
             .as_str(),
         ))
@@ -2714,7 +2721,7 @@ async fn env_var_overrides_loaded_auth() {
         name: "custom".to_string(),
         base_url: Some(format!("{}/openai", server.uri())),
         // Reuse the existing environment variable to avoid using unsafe code
-        env_key: Some(existing_env_var_with_random_value.to_string()),
+        env_key: Some(EXISTING_ENV_VAR_WITH_NON_EMPTY_VALUE.to_string()),
         query_params: Some(std::collections::HashMap::from([(
             "api-version".to_string(),
             "2025-04-01-preview".to_string(),