fix(logs): use singular wording when clearing one old log file

[694410194]

Summary

• use singular wording when cortex logs --clear removes exactly one old log file
• keep plural wording for every other count
• add focused unit coverage for the label helper

Verification

• git diff --check
• rustc --version (not available in this environment, so cargo-based tests could not be run here)

Fixes #43648
/claim #43648

[greptile-apps]

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