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/core/tests/suite/exec.rs

@@ -32,7 +32,7 @@ async fn run_test_cmd(tmp: TempDir, cmd: Vec<&str>) -> Result<ExecToolCallOutput
     let params = ExecParams {
         command: cmd.iter().map(ToString::to_string).collect(),
         cwd: tmp.path().to_path_buf(),
-        timeout_ms: Some(1000),
+        expiration: 1000.into(),
         env: HashMap::new(),
         with_escalated_permissions: None,
         justification: None,