fix: _rstrip_last_assistant_message only strips trailing whitespace from the content — it does not remove the assistant message.

[wooyongbin3-cpu]

Auto-generated fix for #7768\n\n### What happened?

Describe the bug
_rstrip_last_assistant_message only strips trailing whitespace from the content — it does not remove the assistant message. So when the message history genuinely ends with a non-empty assistant message, it passes through unchanged and Anthropic rejects it.

Claude Recommend Fix

        from autogen_ext.models.anthropic import AnthropicChatCompletionClient

        class _AnthropicClient(AnthropicChatCompletionClient):
            # autogen-ext 0.7.x only whitespace-strips trailing assistant messages;
            # Anthropic API requires the conversation to end with a user message.
            def _rstrip_last_assistant_message(self, messages: Sequence[_LLMMessage]) -> Sequence[_LLMMessage]:
                msgs = list(messages)