Fix Anthropic requests ending with assistant messages

[jdoughty04]

Why are these changes needed?

Anthropic rejects requests when the final conversation turn is an assistant message. The current _rstrip_last_assistant_message helper only strips trailing whitespace from that message, so a non-empty trailing assistant response still gets sent to Anthropic unchanged.

This updates the helper to drop trailing assistant messages before request conversion. It also avoids mutating the caller's original message history and adds coverage for both consecutive trailing assistant messages and assistant messages that appear earlier in the conversation.

Related issue number

Closes #7768

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

Local validation:

• uv run --extra anthropic pytest tests/models/test_anthropic_model_client.py -q
• uv run ruff format --check src/autogen_ext/models/anthropic/_anthropic_client.py tests/models/test_anthropic_model_client.py
• uv run ruff check src/autogen_ext/models/anthropic/_anthropic_client.py tests/models/test_anthropic_model_client.py
• uv run pyright src/autogen_ext/models/anthropic/_anthropic_client.py tests/models/test_anthropic_model_client.py
• uv run python -m py_compile src/autogen_ext/models/anthropic/_anthropic_client.py
• git diff --check

[jdoughty04]

@microsoft-github-policy-service agree