fix: add type annotations to AdCPBaseModel methods

Add proper type annotations to model_dump() and model_dump_json()
to satisfy mypy strict type checking.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: bokelley

src/adcp/types/base.py

@@ -2,6 +2,8 @@
 
 """Base model for AdCP types with spec-compliant serialization."""
 
+from typing import Any
+
 from pydantic import BaseModel
 
 
@@ -13,12 +15,12 @@ class AdCPBaseModel(BaseModel):
     when not present (not sent as null).
     """
 
-    def model_dump(self, **kwargs):
+    def model_dump(self, **kwargs: Any) -> dict[str, Any]:
         if "exclude_none" not in kwargs:
             kwargs["exclude_none"] = True
         return super().model_dump(**kwargs)
 
-    def model_dump_json(self, **kwargs):
+    def model_dump_json(self, **kwargs: Any) -> str:
         if "exclude_none" not in kwargs:
             kwargs["exclude_none"] = True
         return super().model_dump_json(**kwargs)