feat(anthropic): accept base_url for OpenAI-compat proxies

Forward base_url through to anthropic.AsyncAnthropic so downstream
consumers can route to Anthropic-compat gateways (e.g. DeepSeek's
/anthropic surface) without monkey-patching the SDK client.

Discovered while diagnosing why cubebox's prompt-cache E2E failed
against DeepSeek: the provider silently connected to api.anthropic.com
with a DeepSeek key. With base_url forwarded, the cache test passes
(cache_read 1900+/2000 on turn 2).

Author: xfgong

cubepi/providers/anthropic.py

@@ -62,12 +62,16 @@ def __init__(
         self,
         *,
         api_key: str | None = None,
+        base_url: str | None = None,
         cache_retention: CacheRetention = "short",
         cache_policy: CacheMarkerPolicy | None = None,
     ) -> None:
         import anthropic
 
-        self._client = anthropic.AsyncAnthropic(api_key=api_key)
+        kwargs: dict[str, Any] = {"api_key": api_key}
+        if base_url is not None:
+            kwargs["base_url"] = base_url
+        self._client = anthropic.AsyncAnthropic(**kwargs)
         self._cache_retention = cache_retention
         self._cache_policy: CacheMarkerPolicy = (
             cache_policy or DefaultCacheMarkerPolicy()