refactor: inline sandbox type lookup in process_exec_tool_call (#7122)

`process_exec_tool_call()` was taking `SandboxType` as a param, but in
practice, the only place it was constructed was in
`codex_message_processor.rs` where it was derived from the other
`sandbox_policy` param, so this PR inlines the logic that decides the
`SandboxType` into `process_exec_tool_call()`.



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* #7112
* __->__ #7122

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

@@ -41,7 +41,7 @@ async fn run_test_cmd(tmp: TempDir, cmd: Vec<&str>) -> Result<ExecToolCallOutput
 
     let policy = SandboxPolicy::new_read_only_policy();
 
-    process_exec_tool_call(params, sandbox_type, &policy, tmp.path(), &None, None).await
+    process_exec_tool_call(params, &policy, tmp.path(), &None, None).await
 }
 
 /// Command succeeds with exit code 0 normally