fix: validate Message objects in validate_messages

[cursor]

Summary

Fixes a validation inconsistency where pre-built Message objects bypassed all checks in validate_messages(), while equivalent dict inputs were fully validated.

Branch: darshan/validate-message-objects

Problem

When callers passed Message instances directly, they were appended without validation. This allowed invalid messages (e.g. empty user content) to reach provider APIs, while the same content as a dict would raise ValidationError.

Solution

Convert Message objects to dicts via model_dump() and route them through the existing dict validation path, ensuring consistent behavior regardless of input type.

Tests

Added tests/test_validators.py covering:

• Empty and whitespace-only user content rejection for Message objects
• Tool role messages with empty content (allowed)
• Assistant messages with tool_calls and empty content (allowed)
• Mixed dict/Message input

CI

All checks pass locally:

• ruff check justllms/
• black --check justllms/
• mypy justllms/ --ignore-missing-imports
• pytest tests/ -v (102 tests)
• python3 -m build