Fix literal backup filename variant grouping

[Dj-Shortcut]

Fixes an edge case where a file literally named backup.jpg was parsed as a backup variant with an empty group key. Adds a regression test and keeps existing img-backup.jpg behavior intact.

Tested:

• python -m pytest faststack/tests/test_variants_logic.py -q

Summary by CodeRabbit

• Bug Fixes

  • Tightened backup file suffix detection patterns to improve accuracy in identifying and correctly mapping variant files within the system, preventing false positives with backup-related filenames.

• Tests

  • Added test coverage for edge case scenarios involving backup-related filenames to ensure correct variant group mapping and proper handling of backup file paths.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 2d1e2a4f-daa9-46f6-8671-79cd04e9e24e

📥 Commits

Reviewing files that changed from the base of the PR and between aa63f27 and 7ed97a8.

📒 Files selected for processing (2)

• faststack/io/variants.py
• faststack/tests/test_variants_logic.py

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Walkthrough

The backup suffix detection regex pattern _BACKUP_TRAILING_RE was refined to match only file stems ending with -backup or -backupN (where N represents optional digits), narrowing its scope from previous logic. A unit test was added to verify the edge case where a filename is exactly backup.jpg.

Changes

Cohort / File(s)	Summary
Backup Regex Pattern Tightening faststack/io/variants.py	Modified _BACKUP_TRAILING_RE to restrict matches to stems with trailing -backup or -backupN suffix only, eliminating broader match locations from the previous pattern.
Edge Case Test Coverage faststack/tests/test_variants_logic.py	Added test for the backup.jpg filename edge case, verifying it maps to the "backup" group with correct main_path and empty backup_paths.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'Fix literal backup filename variant grouping' clearly summarizes the main change: fixing an edge case where files literally named 'backup.jpg' were incorrectly parsed as backup variants with empty group keys.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[AlanRockefeller]

Suggestions for this pull request merged into #91

Thanks for your help!