pkg/util/rand helpers panic on invalid input instead of returning errors

Describe the bug

pkg/util/rand provides two helpers — String and StringFromCharset — that both return (string, error). The signature implies invalid inputs should surface as a Go error, but some invalid inputs currently cause the process to panic instead:

• StringFromCharset(1, "") panics in crypto/rand.Int because the upper bound is <= 0.
• String(-1) panics in make([]byte, -1) because the slice length is negative.

Since both functions already declare an error return, callers naturally expect that path to be used. Today they cannot recover — the helpers crash the host process before any caller code can react.

---

Expected behavior

Invalid inputs should return a clear error instead of panicking:

StringFromCharset(1, "")  ->  "", error("rand: cannot generate ... from empty charset")
String(-1)                ->  "", error("rand: requested length -1 is negative")

n == 0 should keep returning ("", nil) regardless of the charset, since that's the natural no-op result for both helpers.

---

Actual behavior

StringFromCharset(1, "") panics with:

panic: crypto/rand: argument to Int is <= 0

String(-1) panics with:

panic: runtime error: makeslice: len out of range

In both cases the process crashes; the error return value is never set.

---

How to Reproduce?

Reproduced locally on the latest master.

Reproducer for empty charset:

package main

import (
    "fmt"

    r "github.com/microcks/microcks-cli/pkg/util/rand"
)

func main() {
    fmt.Println("Testing empty charset...")
    value, err := r.StringFromCharset(1, "")
    fmt.Printf("value=%q err=%v\n", value, err)
}

Output:

Testing empty charset...
panic: crypto/rand: argument to Int is <= 0

Reproducer for negative length:

package main

import (
    "fmt"

    r "github.com/microcks/microcks-cli/pkg/util/rand"
)

func main() {
    fmt.Println("Testing negative length...")
    value, err := r.String(-1)
    fmt.Printf("value=%q err=%v\n", value, err)
}

Output:

Testing negative length...
panic: runtime error: makeslice: len out of range

In both cases the program exits via panic before the err= line can print.

---

Microcks version or git rev

No response

---

Install method (docker-compose, helm chart, operator, docker-desktop extension,...)

No response

---

Additional information

Root cause. StringFromCharset in pkg/util/rand/rand.go does not validate n or charset before allocating the output slice and calling crypto/rand.Int:

func StringFromCharset(n int, charset string) (string, error) {
    b := make([]byte, n)
    maxIdx := big.NewInt(int64(len(charset)))
    for i := 0; i < n; i++ {
        randIdx, err := rand.Int(rand.Reader, maxIdx)
        ...
    }
    ...
}

• n < 0 → make([]byte, n) panics with makeslice: len out of range.
• n > 0 with charset == "" → rand.Int(rand.Reader, big.NewInt(0)) panics with argument to Int is <= 0.
• n == 0 → already works correctly (loop doesn't execute, returns ("", nil)).

String calls StringFromCharset and inherits the negative-length panic.

Proposed fix. Add two guard clauses at the top of StringFromCharset:

if n < 0 {
    return "", fmt.Errorf("rand: requested length %d is negative", n)
}
if n > 0 && len(charset) == 0 {
    return "", fmt.Errorf("rand: cannot generate %d-character string from empty charset", n)
}

This preserves every currently-working input (including n == 0) and turns both panic paths into descriptive errors. The public signatures stay the same, so no callers (including the SSO PKCE flow in cmd/login.go) need to change.

A small test file pkg/util/rand/rand_test.go would lock this in with table-driven cases for both valid and invalid inputs — the package currently has no test coverage.

I would like to send a small PR for this if the maintainers agree on the approach.

Beginne in pkg/util/rand/rand.go und konzentriere dich auf StringFromCharset und den String-Wrapper. Überprüfe anschließend die in diesem Issue vorgeschlagenen Fälle für ungültige Eingaben. Füge pkg/util/rand/rand_test.go mit tabellengesteuerten Tests für negative Längen, leere Zeichensätze, die Länge null und gültige Eingaben hinzu. Als erledigt gilt die Aufgabe, wenn ungültige Eingaben beschreibende Fehler zurückgeben, ohne einen Panic auszulösen, und das bestehende Verhalten bei gültigen Eingaben unverändert bleibt.