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.








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[//]: # (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/src/tasks/user_shell.rs

@@ -95,7 +95,9 @@ impl SessionTask for UserShellCommandTask {
             command: command.clone(),
             cwd: cwd.clone(),
             env: create_env(&turn_context.shell_environment_policy),
-            timeout_ms: Some(USER_SHELL_TIMEOUT_MS),
+            // TODO(zhao-oai): Now that we have ExecExpiration::Cancellation, we
+            // should use that instead of an "arbitrarily large" timeout here.
+            expiration: USER_SHELL_TIMEOUT_MS.into(),
             sandbox: SandboxType::None,
             with_escalated_permissions: None,
             justification: None,