Fix #7768 - Pop trailing empty AssistantMessage instead of rstrip whitespace mutation

[Whning0513]

_rstrip_last_assistant_message only strips trailing whitespace from the content -- it does not remove the assistant message

The _rstrip_last_assistant_message method only whitespace-strips trailing assistant messages instead of removing them entirely. When the message history genuinely ends with a non-empty assistant message, it passes through unchanged and the Anthropic API rejects it (Anthropic requires the conversation to end with a user message).

Reported by: shivase

The original _rstrip_last_assistant_message method checked if the last message was an AssistantMessage with empty/whitespace-only content and only popped it if the content was empty after stripping. However, the Anthropic API requires conversations to end with a user message, and any trailing assistant message (even non-empty) causes a rejection.

File 1: autogen/python/packages/autogen-ext/src/autogen_ext/models/anthropic/_anthropic_client.py

• Updated _rstrip_last_assistant_message() to pop the last message if it is any AssistantMessage (not just empty ones), using a while loop to handle consecutive trailing assistant messages.

File 2: autogen/python/packages/autogen-ext/src/autogen_ext/models/openai/_openai_client.py

• Applied the same fix pattern for Claude model family handling, where _rstrip_last_assistant_message is called for model names starting with "claude-" to ensure proper message ordering.

Low - The change only affects the edge case where messages end with an assistant message and the target model is Anthropic (or Claude via OpenAI-compatible endpoint). For all other cases, behavior remains identical. Removing trailing assistant messages is semantically safe since the model would generate a new response anyway.

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