Promote shared helpers for suite tests (#6460)

## Summary
- add `TestCodex::submit_turn_with_policies` and extend the response
helpers with reusable tool-call utilities
- update the grep_files, read_file, list_dir, shell_serialization, and
tools suites to rely on the shared helpers instead of local copies
- make the list_dir helper return `anyhow::Result` so clippy no longer
warns about `expect`

## Testing
- `just fix -p codex-core`
- `cargo test -p codex-core --test all
suite::grep_files::grep_files_tool_collects_matches`
- `cargo test -p codex-core
suite::grep_files::grep_files_tool_collects_matches -- --ignored`
(filter requests ignored tests so nothing runs, but the build stays
clean)


------
[Codex
Task](https://chatgpt.com/codex/tasks/task_i_69112d53abac83219813cab4d7cb6446)

codex-rs/core/tests/common/test_codex.rs

@@ -159,14 +159,28 @@ impl TestCodex {
     }
 
     pub async fn submit_turn(&self, prompt: &str) -> Result<()> {
-        self.submit_turn_with_policy(prompt, SandboxPolicy::DangerFullAccess)
-            .await
+        self.submit_turn_with_policies(
+            prompt,
+            AskForApproval::Never,
+            SandboxPolicy::DangerFullAccess,
+        )
+        .await
     }
 
     pub async fn submit_turn_with_policy(
         &self,
         prompt: &str,
         sandbox_policy: SandboxPolicy,
+    ) -> Result<()> {
+        self.submit_turn_with_policies(prompt, AskForApproval::Never, sandbox_policy)
+            .await
+    }
+
+    pub async fn submit_turn_with_policies(
+        &self,
+        prompt: &str,
+        approval_policy: AskForApproval,
+        sandbox_policy: SandboxPolicy,
     ) -> Result<()> {
         let session_model = self.session_configured.model.clone();
         self.codex
@@ -176,7 +190,7 @@ impl TestCodex {
                 }],
                 final_output_json_schema: None,
                 cwd: self.cwd.path().to_path_buf(),
-                approval_policy: AskForApproval::Never,
+                approval_policy,
                 sandbox_policy,
                 model: session_model,
                 effort: None,