fix: preserve Godot virtual callbacks in unused-code checks

[nicodes]

Godot invokes virtual callbacks such as _get_configuration_warnings and _get_property_list without script call sites. The usage scanner already recognizes these callbacks, but the unused-code checker kept a shorter duplicate list and could report/remove them.

Use the existing shared callback predicate for both paths. Regression tests cover actual callback analysis, ordinary helper names, and non-function entities. go test ./... passes, and the built linter clears the reproduced callback failure in Revik without relaxing its lint configuration.

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