fix: immediate cancel check after prefill completes (#1575)

## Problem

When a request is cancelled during prefill, the cancellation is not
detected until `check_for_cancel_every` additional tokens have been
generated. This is because `tokens_since_last_cancel_check` is
initialized to `0`, meaning the first cancel check only happens after
generating `check_for_cancel_every` tokens post-prefill.

For long prefills (which are the most likely to be cancelled), this adds
unnecessary latency before the cancellation is actually honoured.

## Fix

Initialize `tokens_since_last_cancel_check` to `check_for_cancel_every`
instead of `0`, so the very first token generated after prefill triggers
an immediate cancel check.

```diff
- tokens_since_last_cancel_check = 0
+ tokens_since_last_cancel_check = check_for_cancel_every
```

## Impact

- Cancellations issued during prefill are detected immediately when
generation begins
- No change in behaviour for non-cancelled requests (the counter resets
to `0` after each check as before)
- 1 line changed

Co-authored-by: rltakashige <[email protected]>

src/exo/worker/runner/llm_inference/runner.py

@@ -313,7 +313,7 @@ def main(
                             mlx_generator = parse_tool_calls(mlx_generator, tool_parser)
 
                         completion_tokens = 0
-                        tokens_since_last_cancel_check = 0
+                        tokens_since_last_cancel_check = check_for_cancel_every
                         for response in mlx_generator:
                             tokens_since_last_cancel_check += 1
                             if tokens_since_last_cancel_check >= check_for_cancel_every: