Change max_tokens type to int to match Anthropic API (strands-agents#588)

Using a string causes the Anthropic API call to fail:
```
anthropic.BadRequestError: Error code: 400 - {'type': 'error', 'error': {'type': 'invalid_request_error', 'message': 'max_tokens: Input should be a valid integer'}}
```

Author: vinc3m1

src/strands/models/anthropic.py

@@ -55,7 +55,7 @@ class AnthropicConfig(TypedDict, total=False):
                 For a complete list of supported parameters, see https://docs.anthropic.com/en/api/messages.
         """
 
-        max_tokens: Required[str]
+        max_tokens: Required[int]
         model_id: Required[str]
         params: Optional[dict[str, Any]]