Fix: drop trailing assistant message when rstrip leaves it empty

[Mayuri (mayuriphad)]

Fixes #7768

Root cause

_rstrip_last_assistant_message() is documented as removing the last assistant message when it is empty, but it only called .rstrip() on the content and left the (now possibly empty) message in the list. When the trailing AssistantMessage.content was whitespace-only, this produced an empty-string content block, which the Anthropic API rejects (text content blocks must be non-empty).

Fix

After stripping, if the content becomes an empty string, drop the message entirely — matching the function's documented behavior. Non-empty trailing assistant messages are still only whitespace-stripped, preserving the existing "prefill" behavior and pre-existing passing tests. Same fix applied to both the Anthropic and OpenAI clients, which share an identical copy of this helper.

Verification

Added regression tests in both test_anthropic_model_client.py and test_openai_model_client.py. Ran the full local suite: all pre-existing and new tests pass (unrelated failures due to missing OPENAI_API_KEY in this environment, confirmed unrelated via traceback inspection).\n\n*(This is a restoration of closed PR #8029)*

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