refactor: remove negative condition when choosing decode or prefill

[rauletorresc]

Improve readability by:

• Removing the negative condition when choosing between decode or prefill steps.
• Removing the else block when returning from inside the if block.

[gemini-code-assist]

Code Review

The changes in xllm/core/runtime/speculative_worker_impl.cpp refactor the step function to improve readability by removing the else block and the negation in the conditional check. The logic now explicitly handles the decode case first and returns, falling through to the prefill case otherwise. As there were no review comments provided for this pull request, I have no feedback to provide.