docs: clarify getDerivedClasses behavior regarding unloaded referencing schemas (#8047)

[ashmitKmishra]

Fixes #8047.

Analysis: getDerivedClasses relies on the current SchemaContext to identify subclasses. Since SchemaContext does not maintain a reverse-index of all possible external schemas that might reference a base class, it is architecturally impossible for this method to return classes from unloaded schemas without a global pre-scan.

Changes:

1. Added a reproduction test case in core/backend demonstrating that getDerivedClasses returns empty until the referencing schema is explicitly loaded.
2. Updated the JSDoc for getDerivedClasses in core/ecschema-metadata to explicitly warn users that referencing schemas must be loaded into the context to be discovered.

This resolves the ambiguity noted by the reporter.

[CLAassistant]

All committers have signed the CLA.

[ColinKerr]

I turned the suggestion from @rschili and @MichaelSwigerAtBentley into a suggested code change. I have approved pending the resolution of that suggestion.

We may leave the issue opened to track the problem for a future fix. But in the meantime this update will help many people avoid introducing a tricky bug into the code. So it's a valuable update to the codebase.

Thanks @ashmitKmishra!