Plugin fails when annotation processor path contains a classes directory instead of a jar

**Plugin version**
3.7.0

**Gradle version**
9.4.1

**JDK version**
21.0.2

Full repro: https://github.com/odenix/analysis-plugin-bug-repro
README includes reproduction steps.

Start with the README and reproduction steps in the linked analysis-plugin-bug-repro repository, then trace how the plugin handles the annotation processor path. Reproduce the failure with a classes directory instead of a jar and identify the plugin entry point responsible. Done means the reproduction succeeds for that path and the existing behavior remains intact.

Written by the indexing model from the issue text.