fix: handle String() round-trip in Set() for stringToString

[happysnaker]

Summary

Fixes #413

String() wraps the output in [...], but Set() was not stripping those brackets before parsing. This caused round-trip failures when the output of String() (e.g., [x=[]]) was passed back to Set(), resulting in keys like '[x' instead of 'x'.

Reproduction

fs := pflag.NewFlagSet("test", pflag.PanicOnError)
fs.StringToString("testflag", map[string]string{}, "")
fs.Set("testflag", "x=[]")
result, _ := fs.GetStringToString("testflag")
// Before: map[[x:[]]  (incorrect)
// After:  map[x:[]]     (correct)

Changes

• Added [...] bracket stripping in Set() at the start of parsing, so the method can handle its own String() output

[happysnaker]

Superseded by existing #414 which addresses the same issue. Closing in favor of the earlier PR.