fix: preserve empty string message content in chat requests

[r7mekmy4g67w6l]

Summary

• _copy_messages previously filtered with truthiness (if v), so empty-string content was dropped.
• Tool / assistant messages with content="" lost their content field before the request was sent.
• Keep all non-None fields; add a regression test.

Why

Tool-calling loops often append tool results that can be an empty string. Dropping content changes the payload sent to /api/chat and can confuse models that expect a tool message to always include content.

Test plan

• Manual assertion that empty content survives _copy_messages and ChatRequest.model_dump(exclude_none=True)
• Added est_copy_messages_preserves_empty_string_content