Add simple client CPU-only dry run

[Travor278]

Description

Adds a CPU-only dry-run policy server for examples/simple_client and an optional --show-action-chunk client flag. This lets users validate the websocket path, fake observation payloads, and returned actions chunk shape before downloading checkpoints, using a GPU, or connecting robot hardware.

The existing simple client defaults are unchanged. The fake server is an example-only utility and does not affect model loading or real policy serving.

Related Issue

Related to #828 and #826.

Motivation and Context

Several users are trying to understand the remote policy server/client workflow, including which config/checkpoint pair to serve and what a basic action chunk response looks like. A lightweight dry run makes it easier to separate websocket/client setup issues from model checkpoint, GPU, or robot deployment issues.

How Has This Been / Can This Be Tested?

Tested locally on Windows with Anaconda py311 / Python 3.11.15 using the simple-client dependencies.

D:\Dev\conda-envs\py311\python.exe -m py_compile examples/simple_client/main.py examples/simple_client/fake_policy_server.py
uvx ruff check examples/simple_client/main.py examples/simple_client/fake_policy_server.py
uvx ruff format --check examples/simple_client/main.py examples/simple_client/fake_policy_server.py

Smoke-tested the dry-run websocket path:

D:\Dev\conda-envs\py311\python.exe examples/simple_client/fake_policy_server.py --host 127.0.0.1 --port 8766 --action-horizon 4 --action-dim 3
D:\Dev\conda-envs\py311\python.exe examples/simple_client/main.py --host 127.0.0.1 --port 8766 --env DROID --num-steps 2 --show-action-chunk --max-action-rows 2

The client printed actions shape=(4, 3) and timing stats for the fake server response.

Checklist

• Added a CPU-only example path that does not require model weights or GPU access.
• Documented both the fake-server dry run and the real policy:checkpoint server command.
• Preserved existing simple-client defaults and real policy behavior.
• Ran focused syntax, lint, format, and websocket smoke checks.