fix: honour deprecated ParseErrorsWhitelist.UnknownFlagsHandling

[lenamonj]

getUnknownFlagsHandling checks the deprecated ParseErrorsWhitelist after the current ParseErrorsAllowlist, but the deprecated branch returns the Allowlist field:

// then, check deprecated ParseErrorsWhitelist:
if f.ParseErrorsWhitelist.UnknownFlagsHandling != ErrorOnUnknownFlag {
    return f.ParseErrorsAllowlist.UnknownFlagsHandling   // <- wrong struct
}

Control only reaches that line when ParseErrorsAllowlist.UnknownFlagsHandling == ErrorOnUnknownFlag, so it always returns ErrorOnUnknownFlag and setting the deprecated field does nothing at all. Callers still on ParseErrorsWhitelist.UnknownFlagsHandling silently lose the behaviour they asked for:

f := pflag.NewFlagSet("t", pflag.ContinueOnError)
f.ParseErrorsWhitelist.UnknownFlagsHandling = pflag.IgnoreUnknownFlag
f.Parse([]string{"--unknown-flag", "value"})   // returns "unknown flag: --unknown-flag"

The neighbouring ParseErrorsWhitelist.UnknownFlags bool is handled correctly, and TestIgnoreUnknownFlagsBackwardsCompat covers it - only the UnknownFlagsHandling field is affected, which is why it slipped through.

One identifier changed. The test follows the existing pattern: testParseWithUnknownFlagsAndPassToArgs takes an optional setup func so the same body can be driven from either struct, and TestIgnoreUnknownFlagsAndPassToArgsBackwardsCompat mirrors the existing TestIgnoreUnknownFlagsBackwardsCompat. With the fix reverted that test fails; with it, the suite is green.

[CLAassistant]

All committers have signed the CLA.

[tomasaschan]

Thank you!