fix: treat an existing file path as literal instead of a glob pattern

[Eljees]

Issue #, if available: Fixes #2136

Description of changes:

ConfigMixIn expands every template path through glob.glob(filename, recursive=True). When the path is a real file whose name contains glob metacharacters, the brackets are read as a character class instead of literal text: the glob matches nothing and the template is silently dropped, or the pattern blows up on a bad character range. The reported case is the file name pytest's snapshot plugin produces:

emr/tests/__snapshots__/test_snapshot[emr--tests--emr.template.2.yaml].yaml

The fix checks the literal path first — if Path(filename).is_file() it is used as-is, otherwise the existing glob expansion runs unchanged. Patterns keep working exactly as before; a path that already resolves to a file is simply never reinterpreted as a pattern.

Test: test_config_literal_path_with_glob_characters creates template[1].yaml in a temporary directory and asserts it survives into config.templates. It fails on main (the list comes back empty) and passes with the fix.

AI-assisted (LLM used for drafting and for running the checks); the analysis and the runs are mine.

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[Eljees]

Ping — this has been open since 6 August with no review yet.

ConfigMixIn expands every template path through glob.glob(filename, recursive=True), so a path that is a real file whose name contains glob metacharacters gets read as a character class: the glob matches nothing and the template is silently dropped, or it fails on a bad character range. The reported case is the filename pytest's snapshot plugin produces, test_snapshot[emr--tests--emr.template.2.yaml].yaml. The fix checks Path(filename).is_file() first and otherwise runs the existing glob expansion unchanged, so patterns behave exactly as before.

No CI has run on it: the check suite for e54c80e is at action_required with zero runs, so it needs a maintainer to approve the workflow. The branch is also behind main by now — happy to rebase.

Happy to close it if you would rather fix #2136 another way.