fix(rand): return errors for invalid random string inputs

[hazelmayank]

This PR fixes two panic cases in pkg/util/rand.

The helper functions String and StringFromCharset return (string, error), but a couple of invalid inputs were causing the process to panic instead of returning an error.

The panic cases were:

• StringFromCharset(1, "") panicked because crypto/rand.Int was called with a max value of 0
• String(-1) panicked because it tried to allocate a byte slice with a negative length

This PR adds input validation before allocation and random index generation, so invalid inputs now return clear errors instead of crashing.

Fixes #359

Valid behavior is preserved:

String(0) -> "", nil
String(n) -> string with length n, nil
StringFromCharset(0, "") -> "", nil
StringFromCharset(10, "abc") -> 10-character string using only a/b/c, nil

I also added unit tests for both valid and invalid inputs.



Test plan
 gofmt -l pkg/util/rand/rand.go pkg/util/rand/rand_test.go
 git diff --check -- pkg/util/rand/rand.go pkg/util/rand/rand_test.go
 go test -v ./pkg/util/rand/...
 make build-local

Note: I did not include go test -cover because my local Go toolchain reports go: no such tool "covdata". This PR is scoped only to pkg/util/rand and its tests.