feat: update process_exec_tool_call() to take a cancellation token (#6972)

This updates `ExecParams` so that instead of taking `timeout_ms:
Option<u64>`, it now takes a more general cancellation mechanism,
`ExecExpiration`, which is an enum that includes a
`Cancellation(tokio_util::sync::CancellationToken)` variant.

If the cancellation token is fired, then `process_exec_tool_call()`
returns in the same way as if a timeout was exceeded.

This is necessary so that in #6973, we can manage the timeout logic
external to the `process_exec_tool_call()` because we want to "suspend"
the timeout when an elicitation from a human user is pending.








---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/openai/codex/pull/6972).
* #7005
* #6973
* __->__ #6972

codex-rs/linux-sandbox/tests/suite/landlock.rs

@@ -40,7 +40,7 @@ async fn run_cmd(cmd: &[&str], writable_roots: &[PathBuf], timeout_ms: u64) {
     let params = ExecParams {
         command: cmd.iter().copied().map(str::to_owned).collect(),
         cwd,
-        timeout_ms: Some(timeout_ms),
+        expiration: timeout_ms.into(),
         env: create_env_from_core_vars(),
         with_escalated_permissions: None,
         justification: None,
@@ -143,7 +143,7 @@ async fn assert_network_blocked(cmd: &[&str]) {
         cwd,
         // Give the tool a generous 2-second timeout so even slow DNS timeouts
         // do not stall the suite.
-        timeout_ms: Some(NETWORK_TIMEOUT_MS),
+        expiration: NETWORK_TIMEOUT_MS.into(),
         env: create_env_from_core_vars(),
         with_escalated_permissions: None,
         justification: None,