[FIXED] Accept {{left}} and {{right}} mapping functions in ValidateMapping - #8568

Problem

The left and right mapping functions were added to the subject transform in #5337 and announced in the 2.10.16 release notes, but the allowlist in ValidateMapping (server/sublist.go) was never updated. It still lists the other eight functions, so a destination using {{left(...)}} or {{right(...)}} is rejected with invalid mapping destination: unknown function.

That blocks them on the path they were added for. #5337's own description uses account-mapping config syntax (foo.*: foo.{{Right(1,2)}}), and that config does not load:

Error adding mapping for "foo.*" to "bar.{{right(1,2)}}" : invalid mapping destination: unknown function in {{right(1,2)}}

The server refuses to start. The same applies to Account.AddMapping and to a stream subject_transform, mirror or source transform, which fail with JSStreamTransformInvalidDestinationError (10156).

Meanwhile RePublish and nats server mapping call NewSubjectTransform directly and accept the very same destination, and the transform itself works correctly (foo.1234 -> bar.34). So the function is implemented and shipped, just not reachable from config.

Change

Add the two regexes to the allowlist, the same way random was added in #6950 and split/slice in #3305. Two lines.

Tests: the left/right cases in TestValidateDestinationSubject (I also added slice, split and random there, which were untested), and TestAccountLeftRightMapping, which loads a config with both functions and checks the mapped subjects arrive as bar.34 and qux.123. Both fail on main.

One alternative worth mentioning: the allowlist looks redundant now. Since #5617 ValidateMapping ends with a NewSubjectTransform call, and indexPlaceHolders already returns ErrUnknownMappingDestinationFunction for an unknown function, so removing the eight regexes would give the same errors from a single source of truth and this class of drift could not happen again. I went with the smaller change to match precedent, but happy to switch if you prefer that.

Not addressed here: left/right mappings pushed through JWT claims currently disappear silently because AddWeightedMappings' error is discarded (#8468 / #8473). That is a separate defect.