Community task: Additional job URL validation test coverage

[codethor0]

Background

Issue #1 established the offline job URL validation lane. Additional ATS providers and edge cases still benefit from parametrized tests.

Scope

• Extend backend/tests/test_job_url_validation.py with additional fixtures
• Cover normalization edge cases and provider detection for approved offline helpers only
• No network calls

Acceptance criteria

• New tests pass locally
• No weakening of existing assertions
• Lookalike-host negative tests preserved or extended

Test / validation

python3.11 -m pytest backend/tests/test_job_url_validation.py -q

Non-goals

• No API route wiring
• No schema changes
• No new dependencies without approval

Privacy constraints

• Use synthetic URLs only

Approval-required boundaries

• Do not change validation semantics broadly without maintainer review