fix: guard enclosure against None/non-positive length values

[Zerthick]

Description

Guard <enclosure> element against None or non-positive length values in the RSS feed.

When a remote image returns 404, the length field becomes None, which would produce invalid XML like <enclosure length="None" />. This PR adds a guard to skip the enclosure element when length is None or non-positive.

Changes

• Add guard condition in rss.xml.jinja2 template: check that item.image[2] is not None and is greater than 0
• Add test verifying no invalid enclosure length values in output

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Split from: #451

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💡 AI Contribution Level: 8 — Based on the VisiData AI contribution scale, this was an AI-assisted contribution where the AI performed the code splitting and PR creation with minimal human guidance.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.16%. Comparing base (1b5e630) to head (1bf72fe).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #457   +/-   ##
=======================================
  Coverage   83.16%   83.16%           
=======================================
  Files          11       11           
  Lines         784      784           
  Branches      133      133           
=======================================
  Hits          652      652           
  Misses         86       86           
  Partials       46       46           

Flag	Coverage Δ
unittests	83.16% <ø> (ø)

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