Fix six race conditions when a user switches or cancels a model while a
previous load or generation is still in flight, across the inference
orchestrator and the /load and /unload routes:
- Cancel an in-flight generation on a safetensors/MLX model switch and
serialize unload with load under the inference lifecycle gate.
- Cancel an in-flight load off the lifecycle gate so a Stop-loading
cancel does not wait out the multi-minute load; guard the dispatched
mailbox against a racing unload.
- Recheck the loading marker after spawn and again after the load
response before publishing, so a load cancelled mid-flight is reaped
instead of going live.
- Discard the loading marker before tearing the subprocess down in
cancel_load, closing a spawn-after-cancel window and an orphaned
compare-mode dispatcher during unload.
- Match the unload target before canceling an in-flight GGUF load and
add an off-gate fast path for the still-loading GGUF case.
- Run the Unsloth unload off the event loop so a paused SSE stream
holding _gen_lock cannot block the loop.
Adds studio/backend/tests/test_orchestrator_unload_cancel.py covering
the unload/cancel/switch race paths.
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