When IP_PROXY_PROVIDER_NAME in config.py is set to an unrecognised
value (e.g. typo "kuaidalli" instead of "kuaidaili"), IpProxyProvider.get()
returns None which is silently stored in ProxyIpPool.ip_provider. The
crash surfaces later as:
AttributeError: 'NoneType' object has no attribute 'get_proxy'
with no hint about what went wrong or how to fix it.
Add an explicit None-check in create_ip_pool() that raises ValueError
with the bad name and the list of valid options, so the root cause is
immediately visible. Also add a regression test covering the four
invalid-name edge cases (typo, empty string, wrong case, unknown name)
and the three valid names to confirm they are unaffected.
