feat:(backend): Implement Context Cancellation

[gphorvath]

Type: Feature

Description: Cancelling context cancels generation to free resources to retry requests or otherwise stop inference.

User Story:
For each backend (VLLM, Embeddings, Llama-cpp-python, Whisper), implement any changes from the SDK required to perform context cancellation.

Acceptance Criteria:

• Implement any downstream changes from the SDK. See feat(sdk): context cancellation #279