fix: prevent engine_destroy crash on stale task handle

[anujdeshpande]

Summary

Fixes #85.

engine_destroy() could crash with a LoadProhibited exception (vTaskDelete → uxListRemove) because both the engine task and engine_destroy() raced to delete the same task:

• engine_task() ends with vTaskDelete(NULL) (self-delete).
• engine_destroy() waited a fixed vTaskDelay(100ms) and then called vTaskDelete(eng->task_handle).

If the task happened to exit during that 100 ms window — common after a server-side disconnect, where in-flight reconnect/timer/peer events wake the task blocked on xQueueReceive(portMAX_DELAY) — it self-deleted first, leaving eng->task_handle stale. engine_destroy() then called vTaskDelete() on the freed handle and crashed, which made it impossible to safely free room resources and leaked ~100 KB of internal RAM per session.

Fix

Replace the fixed delay + unconditional delete with an explicit join:

• Add a binary "task done" semaphore, created before the task so the task can always signal completion.
• The task gives the semaphore right before vTaskDelete(NULL).
• engine_destroy() enqueues a new internal _EV_STOP event to wake the (possibly blocked) task so it observes is_running == false and exits promptly, then waits on the semaphore.
• The handle is only force-deleted as a last resort if the task fails to exit within ENGINE_TASK_JOIN_TIMEOUT_MS (5 s).

This removes the double-delete entirely: in the normal path only the task deletes itself, and engine_destroy() just joins and clears the handle.

_EV_STOP is ignored by every state handler (default: break; → event freed), so waking the task has no side effects on the state machine.

Testing

• Built the minimal example for esp32s3 with ESP-IDF v5.5.1 — compiles cleanly with no warnings on engine.c, image created successfully.
• Verified control flow and ordering by inspection: the semaphore is created before the task; the task is joined before the queue/timer/signal/peers are torn down; _EV_STOP is a safe no-op for all states.

The change is contained to components/livekit/core/engine.c.

[CLAassistant]

All committers have signed the CLA.

[ladvoc]

@anujdeshpande, thank you for your contribution—I will take a look.

[ladvoc]

LGTM ✅, this will be included in the next release.