feat(polars): expose validation errors

[bnkc]

Summary

• add a nullable error field to Polars validation results
• preserve the exact Python-facing validation message for syntax and value errors
• document extracting the field as an email_status column
• add regression coverage for valid, invalid, overlong, and null inputs
• refresh CI inputs for current Rust and officially supported free-threaded Python builds

Tests

• cargo test --lib (163 passed)
• uv run --python 3.13 --all-extras pytest -q (114 passed)

Closes #33