Extract the response delay and timeout simulation into a library

[adamtheturtle]

MockVWS._wrap_callback in src/mock_vws/decorators.py and _make_respx_callback in src/mock_vws/_respx_mock_server/decorators.py simulate a slow server and raise the correct client-side timeout for both requests and httpx. Neither responses nor respx supports this, so anyone testing client timeout handling today either monkeypatches or runs a real slow server.

Getting it right needs knowledge which is not discoverable from the libraries' documentation:

• responses attaches req_kwargs to PreparedRequest dynamically, so it is absent from the requests type stubs.
• requests accepts the timeout as a (connect, read) tuple, and only the read leg is meaningful for a server which is slow to respond.
• httpx puts the equivalent in request.extensions["timeout"].
• Each needs a different exception raised: requests.exceptions.Timeout and httpx.ReadTimeout.

The injectable sleep_fn is also part of the value, because it lets callers control virtual time instead of really sleeping.

This built on #2884, #2919 and #2974. Closing this means the code moves to its own package and this repository depends on it.