Accept at-signs inside quoted email local parts

[prnvh]

The email validator rejects valid quoted local parts containing @, such as "test@test"@example.com, because its early guard requires exactly one at-sign in the entire address.

RFC 5321 section 4.1.2 permits @ inside a quoted local part. The function already splits at the final at-sign and validates the complete local part and domain separately, so the guard only needs to require that a separator is present.

This change:

• Allows at-signs inside quoted local parts, including multiple at-signs.
• Moves the existing "test@test"@example.com fixture from invalid to valid cases.
• Adds regression tests for quoted at-signs and malformed addresses, including unquoted extra separators, unmatched quotes, missing domains and invalid domains.

Standard reference: https://www.rfc-editor.org/rfc/rfc5321#section-4.1.2

Validation on Python 3.12.14:

• Before the fix, the email regression suite has four failures and 40 passes.
• With the fix, pytest . -q: 1,182 passed.
• tox -e py312: passed, including a fresh source-distribution build and 1,182 tests.
• ruff check . and ruff format --check .: passed.
• pyright --pythonpath .venv/Scripts/python.exe .: zero errors or warnings.
• bandit -c pyproject.toml -r src/validators: no issues identified.