chore: sanbox extraction (#4286)

# Extract and Centralize Sandboxing
- Goal: Improve safety and clarity by centralizing sandbox planning and
execution.
  - Approach:
- Add planner (ExecPlan) and backend registry (Direct/Seatbelt/Linux)
with run_with_plan.
- Refactor codex.rs to plan-then-execute; handle failures/escalation via
the plan.
- Delegate apply_patch to the codex binary and run it with an empty env
for determinism.

codex-rs/core/src/executor/mod.rs

@@ -0,0 +1,64 @@
+mod backends;
+mod cache;
+mod runner;
+mod sandbox;
+
+pub(crate) use backends::ExecutionMode;
+pub(crate) use runner::ExecutionRequest;
+pub(crate) use runner::Executor;
+pub(crate) use runner::ExecutorConfig;
+pub(crate) use runner::normalize_exec_result;
+
+pub(crate) mod linkers {
+    use crate::codex::ExecCommandContext;
+    use crate::exec::ExecParams;
+    use crate::exec::StdoutStream;
+    use crate::executor::backends::ExecutionMode;
+    use crate::executor::runner::ExecutionRequest;
+
+    pub struct PreparedExec {
+        pub(crate) context: ExecCommandContext,
+        pub(crate) request: ExecutionRequest,
+    }
+
+    impl PreparedExec {
+        pub fn new(
+            context: ExecCommandContext,
+            params: ExecParams,
+            approval_command: Vec<String>,
+            mode: ExecutionMode,
+            stdout_stream: Option<StdoutStream>,
+            use_shell_profile: bool,
+        ) -> Self {
+            let request = ExecutionRequest {
+                params,
+                approval_command,
+                mode,
+                stdout_stream,
+                use_shell_profile,
+            };
+
+            Self { context, request }
+        }
+    }
+}
+
+pub mod errors {
+    use crate::error::CodexErr;
+    use crate::function_tool::FunctionCallError;
+    use thiserror::Error;
+
+    #[derive(Debug, Error)]
+    pub enum ExecError {
+        #[error(transparent)]
+        Function(#[from] FunctionCallError),
+        #[error(transparent)]
+        Codex(#[from] CodexErr),
+    }
+
+    impl ExecError {
+        pub(crate) fn rejection(msg: impl Into<String>) -> Self {
+            FunctionCallError::RespondToModel(msg.into()).into()
+        }
+    }
+}