[Core] Make NPCStepper.generate_local() sync in local mode

[A-Mahla]

Change NPCStepper.get_action() NPCStepper.generate_local() to synchronous function. The README.md and tests/test_step.py files are updated accordingly.

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[tristandeborde]

After thinking about it, let's keep get_action() async; otherwise lots of parallel calls from Unity to a local API (which I'm currently setting), would block the server while waiting for the LLM's answer.

[tristandeborde]

LGTM