feat: [DGP-1824] set a targetFile fallback when TargetFileFromPlugin is empty

[bsalomon-snyk]

Set a targetFile fallback for pip when TargetFileFromPlugin is empty

• fall back to normalisedTargetFile when targetFileFromPlugin is absent

• Tests written and linted ℹ︎

• [N/A] Documentation written ℹ︎

• Commit history is tidy ℹ︎

What this does

For SCA Monitor we receive an empty target_file for a few environments:

• npm/yarn/pnpm -- snyk-nodejs-lockfile-parser -- plugin metadata has no targetFile
• python (requirements.txt / pip) -- snyk-python-plugin -- returnedTargetFile() returns undefined for the default case
• Gradle (build.gradle) -- snyk-gradle-plugin -- explicitly returns undefined for non-.kts (BST-529 workaround)

This changes sets pip to use the normalizedTargetFile (requirements.txt). The other ecosystems have defined reasons for not doing so.

[snyk-io]

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[snyk-pr-review-bot]

PR Reviewer Guide 🔍

🧪 PR contains tests

🔒 No security concerns identified

⚡ No major issues detected

📚 Repository Context Analyzed This review considered 4 relevant code sections from 4 files (average relevance: 1.00) pkg/ecosystems/python/pip/plugin.go (lines 1-246) pkg/depgraph/sbom_resolution.go (lines 1-231) pkg/ecosystems/python/uv/plugin.go (lines 233-387) pkg/ecosystems/legacy/plugin.go (lines 1-170)