fix: strip trailing whitespace from Anthropic assistant messages

[VANDRANKI]

Fixes #4413.

Bug

Anthropic's API returns a 400 BadRequestError when any assistant message has trailing whitespace in its content. CrewAI sometimes appends partial assistant turns to steer the model (e.g. "content": "Say: "), which can end with a space.

Fix

In _format_messages_for_provider, strip trailing whitespace from assistant message content before returning for Anthropic models:

for msg in messages:
    if msg["role"] == "assistant" and isinstance(msg.get("content"), str):
        sanitized.append({**msg, "content": msg["content"].rstrip()})
    else:
        sanitized.append(msg)

The sanitization is scoped to the Anthropic code path only and does not affect other providers.