fix: keep every enum value when a deduplicated name collides with another - #2669

When two enum values sanitize to the same Go name, SanitizeEnumNames appends a numeric suffix. That suffix could land on a name another value already owns, and the map write silently replaced it. For example enum: [A, a, A1] produced only A and A1, so one value lost its constant (and [A, A1, a] lost A1's original mapping the same way).

The suffix loop now skips any name that another value claims or that is already taken, so the example yields A, A2 and A1. Enums without such a collision keep exactly the names they had before. The new test covers collisions with a later and an earlier value, plus the unchanged no-collision case.