fix: prevent panic in IsDSN on short input strings

[SAY-5]

IsDSN guards s[:11] with len(s) > 11 but then evaluates s[:13] without a length check, so any non-DSN argument that is exactly 12 characters (for example pgdesigner docs/apisrv.) panics with slice bounds out of range [:13] with length 12 instead of being treated as a file path.

This switches the two comparisons to strings.HasPrefix, which does its own bounds checking and reads more clearly. Added a table-driven TestIsDSN covering both schemes and the short-string cases.

Fixes #17