[validation fixture] Add one focused validation fixture

[stdiodh]

Goal

Add one small validation fixture that demonstrates a single existing validation rule.

Context

Career Feed uses fixtures to make validator behavior easy to review. A good fixture should isolate one rule and avoid broad behavior changes.

Suggested work

• Pick one existing validation rule from scripts/validate-career-feed-brief.py.
• Add one fixture under tests/fixtures/ that clearly passes or fails that rule.
• Update the smallest relevant test or validation command if needed.

Acceptance criteria

• The fixture targets one clear rule.
• Existing validation behavior is preserved.
• ./scripts/validate.sh passes.
• The fixture contains no secrets, private URLs, or fake usage metrics.