fix(native-sidecar): Clone cancel descriptor under single VmState borrow

[mittal-parth]

Summary

• CancelExecution set deadline_task via get_mut, then called get() to clone the descriptor while that RefMut was still live.
• This panicked with "RefCell already mutably borrowed".
• This change clones the descriptor from the same get_mut, fixing the panic on abort / cancel.

Tests

All tests under crates/native-sidecar/tests/language_execution.rs now pass

> cargo test -p agentos-native-sidecar --test language_execution -- --test-threads=1
....

running 18 tests
test attached_execution_is_ephemeral_and_does_not_capture_by_default ... ok
test background_and_retained_events_require_valid_identity ... ok
test background_lifecycle_replays_cancels_resets_and_deletes ... ok
test background_process_records_do_not_appear_as_contexts ... ok
test context_survives_completed_operation_ttl ... ok
test contexts_are_explicit_unique_and_language_pinned ... ok
test evaluation_rejects_non_json_values_with_a_structured_result ... ok
test execution_timeout_is_enforced_by_the_sidecar ... ok
test javascript_execution_reuses_retained_context ... ok
test javascript_module_context_shares_state_through_global_this ... ok
test javascript_module_execution_accepts_inline_exports_in_a_context ... ok
test javascript_module_execution_awaits_top_level_promises_in_a_context ... ok
test package_mutations_are_exclusive_across_executions ... ok
test python_execution_reuses_retained_globals ... ok
test stderr_capture_is_independent_from_retained_event_history ... ok
test support::bridge_support::recording_bridge_rejects_symlink_cycles_when_following_metadata ... ok
test support::collect_process_output_stream_append_is_bounded ... ok
test typescript_check_reports_semantic_diagnostics ... ok

test result: ok. 18 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 10.64s

Ref

Closes #1988

[the-company-company]

✅ No issues found

_{Reviewed commit 81ba4e7.}