Raise a source error for anonymous classes

[OskarEichler]

Summary

Normalize anonymous class/module source lookup failures to MethodSource::SourceNotFoundError instead of leaking NameError or TypeError from constant lookup.

Context and fix

class_comment has no constant name to pass to const_source_location for anonymous owners. Detect that boundary explicitly and raise the library error used for other unavailable source locations.

Regression coverage

• added an anonymous-class class_comment regression
• focused and full suites pass on Ruby 4.0.6 and 3.2.11
• final composition: 41 examples / 0 failures on both Rubies
• syntax, diff check, exact packaging and Rails integration pass