fix(proxy): raise ValueError for unknown IP_PROXY_PROVIDER_NAME

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.

proxy/proxy_ip_pool.py

@@ -202,11 +202,17 @@ async def create_ip_pool(ip_pool_count: int, enable_validate_ip: bool) -> ProxyI
     :param enable_validate_ip: Whether to enable IP proxy validation
     :return:
     """
+    ip_provider = IpProxyProvider.get(config.IP_PROXY_PROVIDER_NAME)
+    if ip_provider is None:
+        raise ValueError(
+            f"Unknown proxy provider: '{config.IP_PROXY_PROVIDER_NAME}'. "
+            f"Valid options: {list(IpProxyProvider.keys())}"
+        )
     is_static = config.IP_PROXY_PROVIDER_NAME == ProviderNameEnum.STATIC_PROVIDER.value
     pool = ProxyIpPool(
         ip_pool_count=ip_pool_count,
         enable_validate_ip=False if is_static else enable_validate_ip,
-        ip_provider=IpProxyProvider.get(config.IP_PROXY_PROVIDER_NAME),
+        ip_provider=ip_provider,
     )
     await pool.load_proxies()
     return pool