Sidecar panics with "RefCell already mutably borrowed" when cancelling a running execution

Aborting an in-flight JavaScript execution makes the cancel request panic in the sidecar.

Repro

const vm = await AgentOs.create();
const controller = new AbortController();
setTimeout(() => controller.abort(), 500);
await vm.javascript.execute("await new Promise((r) => setTimeout(r, 60_000))", { signal: controller.signal });

Actual

• The call resolves cancelled, but the cancel request fails with ERR_AGENTOS_REQUEST_TASK_PANIC: … "RefCell already mutably borrowed" at crates/native-sidecar/src/state.rs:1079.
• vm.dispose() then waits out ERR_AGENTOS_VM_TEARDOWN_DEADLINE.
• The same panic fails 2 of 18 tests in crates/native-sidecar/tests/language_execution.rs (background_lifecycle_replays_cancels_resets_and_deletes, package_mutations_are_exclusive_across_executions).

Expected
Cancel completes without a panic, and dispose returns promptly.

Start by reproducing the cancellation case from the JavaScript example, then inspect crates/native-sidecar/src/state.rs:1079 and the failing scenarios in crates/native-sidecar/tests/language_execution.rs. Verify the fix against background_lifecycle_replays_cancels_resets_and_deletes and package_mutations_are_exclusive_across_executions; done means cancellation no longer panics and vm.dispose() returns promptly.