Document `Set` semantics for `selectClasspathResource()`, `selectClasspathRoots()`, and `selectModules()` in `DiscoverySelectors`

[sbrannen]

Overview

The selectClasspathResource(Set<Resource>), selectClasspathRoots(Set<Path>), and selectModules(Set<String>) methods in DiscoverySelectors all accept a Set and return a List.

The Set semantics are intended to enforce uniqueness.

Whereas, the List semantics are intended to enforce order.

However, if the user supplies a Set that does not enforce a particular iteration order, the discovery results can change the effective execution order of the test suite.

Deliverables

• Add notes to the Javadoc for selectClasspathResource(Set<Resource>), selectClasspathRoots(Set<Path>), and selectModules(Set<String>), stating that the supplied Set should ideally be a SequencedSet (on Java 21+), a LinkedHashSet, or similar in order to guarantee reliable selection/execution order.