fix(agent): guard .get(key, "").method() None dereference in adapters

dict.get(key, default) returns None (not the default) when the key
EXISTS with value None. The default only applies when the key is ABSENT.
Chained method calls (.strip(), .upper(), .count()) crash with
AttributeError on NoneType.

Fix two confirmed hits:
- auxiliary_client.py: custom provider base_url/api_key (config null)
- anthropic_adapter.py: text block content (API null response)

Pattern: .get(key, "").method() → (.get(key) or "").method()

agent/anthropic_adapter.py

@@ -2102,7 +2102,7 @@ def _convert_user_message(content: Any) -> Dict[str, Any]:
     if isinstance(content, list):
         converted_blocks = _convert_content_to_anthropic(content)
         if not converted_blocks or all(
-            b.get("text", "").strip() == ""
+            (b.get("text") or "").strip() == ""
             for b in converted_blocks
             if isinstance(b, dict) and b.get("type") == "text"
         ):

agent/auxiliary_client.py

@@ -4716,8 +4716,8 @@ def resolve_provider_client(
         if custom_entry is None:
             custom_entry = _get_named_custom_provider(provider)
         if custom_entry:
-            custom_base = custom_entry.get("base_url", "").strip()
-            custom_key = custom_entry.get("api_key", "").strip()
+            custom_base = (custom_entry.get("base_url") or "").strip()
+            custom_key = (custom_entry.get("api_key") or "").strip()
             custom_key_env = (custom_entry.get("key_env") or custom_entry.get("api_key_env") or "").strip()
             if not custom_key and custom_key_env:
                 custom_key = os.getenv(custom_key_env, "").strip()