Refactor comment filtering

[Vladyslav-Kuksiuk]

This PR extracts common functions used across all comment-filtering mechanisms to reduce code duplication.

[Oleg-Melnik]

Claude Review

Optional cleanup (non-blocking): csharpInterpolation.start = "{" in csharp_filter.go is set but never read — C#'s interpolated-string-text path checks a literal '{' and only openBrace/closeBrace are consumed by consumeInterpolationCodeByte. It's harmless (it documents the struct consistently with the JS/Kotlin interpolation forms, where start is used), so no change is required — just flagging it as dead within the C# path.

[Oleg-Melnik]

@Vladyslav-Kuksiuk LGTM with a minor comment.