Single-rune Unicode flags can be emitted by completion but not parsed #2433

My urfave/cli version is

v3.12.0 (9b981a04295de5121091c479f8a7a0476d2e8d38)

Checklist

• I am running the latest v3 release.
• I checked the v3 manual.
• I searched existing issues and pull requests for this problem.

Dependency Management

• My project is using Go modules.

Describe the bug

Single-rune Unicode flag names are handled inconsistently. Shell completion counts runes and emits a single-dash flag such as -ש, but help formatting counts bytes and displays --ש. More importantly, the parser examines rune(firstArg[1]), which converts only the first UTF-8 byte rather than decoding the first rune.

For ש, the first byte is 0xD7; converting that byte to a rune produces ×, which is not a letter. The parser therefore classifies -ש as a positional argument and never sets the registered flag.

To reproduce

package main

import (
    "context"
    "fmt"

    "github.com/urfave/cli/v3"
)

func main() {
    const name = "ש"
    cmd := &cli.Command{
        Name:  "app",
        Flags: []cli.Flag{&cli.StringFlag{Name: name}},
        Action: func(_ context.Context, cmd *cli.Command) error {
            fmt.Printf("value=%q args=%q\n", cmd.String(name), cmd.Args().Slice())
            return nil
        },
    }
    _ = cmd.Run(context.Background(), []string{"app", "-" + name, "value", "operand"})
}

Observed behavior

value="" args=["-ש" "value" "operand"]

Help formatting also prefixes this single-rune name with --, while flag completion uses -.

Expected behavior

A one-rune Unicode flag should be treated consistently with a one-byte ASCII flag:

value="value" args=["operand"]

Help and completion should both render it as -ש.

Additional context

prefixFor uses len(name) == 1, while completion already uses utf8.RuneCountInString. In parseFlags, decoding the first rune after the leading dash instead of converting firstArg[1] would preserve existing ASCII behavior and correctly classify Unicode letters.

Want to fix this yourself?

Yes. The fix is small and can be covered by a focused regression test.

Run go version and paste its output here

go version go1.26.5 windows/amd64

Relevant go env output

GOOS=windows
GOARCH=amd64
GOVERSION=go1.26.5
CGO_ENABLED=0