Detect conda explicit files by content instead of extension

[pditommaso]

Summary

• Changed conda explicit file detection from extension-based (.txt) to content-based by checking for the @EXPLICIT marker
• The detection now reads the first 20 lines of a file to look for the @EXPLICIT marker, which is the standard identifier for conda explicit specification files
• This approach is more reliable and flexible than relying on file extensions

Motivation

Previously, conda explicit files were identified solely by their .txt extension using isTextFilePath(). This had two issues:

1. False negatives: Conda explicit files with non-standard extensions (e.g., .lock, .spec) were not recognized
2. False positives: Regular text files with .txt extension could be incorrectly treated as conda explicit files

The @EXPLICIT marker is the canonical identifier for conda explicit specification files (see conda documentation). By checking file content, we can reliably identify these files regardless of their extension.

Changes

CondaCache.groovy

• Replaced isTextFilePath() with isExplicitFile() - checks if file exists and contains @EXPLICIT marker
• Added containsExplicitMarker() helper - efficiently reads only first 20 lines
• Updated condaPrefixPath() and createLocalCondaEnv0() to use the new method

CondaCacheTest.groovy

• Updated tests to use actual conda explicit file format with @EXPLICIT marker
• Tests now create real files with proper content rather than relying on extension checks

Test plan

• All existing CondaCacheTest tests pass
• New content-based detection correctly identifies files with @EXPLICIT marker
• Files without @EXPLICIT marker are not incorrectly identified as explicit files
• Non-existent files return false gracefully

🤖 Generated with Claude Code

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[pditommaso]

@ewels does this match the same expectation of #6665?

[ewels]

Yup, LGTM 👍🏻