fix: reject blank chat messages

[attainWei]

Summary

• reject messages containing only whitespace at the active backend request model
• add focused Pydantic validation tests
• preserve surrounding whitespace for valid messages

Verification

• .venv\Scripts\python.exe -m pytest backend\tests\unit\test_chat_request_validation.py -q: 4 passed
• python -m py_compile backend/models.py
• git diff --check

Scope

This is a companion example for the Codex book manuscript. Full application and integration regression suites were not run.