Warn for unknown catchpoint classes

[OskarEichler]

Summary:

• evaluate the requested constant and inspect the resulting object
• warn unless it resolves to a Class
• avoid testing whether the textual boolean result itself is a Class

Why:
The current code evaluates exception.is_a?(Class).to_s through warning_eval, then calls is_a?(Class) on that evaluator result. The condition does not represent the requested constant and valid/invalid class warnings are inverted or lost.

Verification:

• focused known-class and unknown/non-class catchpoint models pass
• current full-suite failure count is unchanged from baseline
• gem build and syntax checks pass

Compatibility:
Catchpoint registration remains unchanged; diagnostics now reflect the evaluated constant.