Allow empty string identifiers on non-flags enums in JsonStringEnumConverter

[Copilot]

[JsonStringEnumMemberName("")] was rejected at runtime for all enums, even though the empty-string ambiguity it was guarding against only applies to [Flags] enums (e.g. "y, , n"). This blocked common interop scenarios where an XSD-derived enum has a member whose XML/JSON representation is the empty string.

public enum YesOrNoOrEmpty
{
    [JsonStringEnumMemberName("y")] Yes,
    [JsonStringEnumMemberName("n")] No,
    [JsonStringEnumMemberName("")]  Empty, // previously threw InvalidOperationException
}

Description

• EnumConverter.ResolveAndValidateJsonName: relaxed the guard so empty strings are permitted on non-flags enums. Flags enums continue to reject empty identifiers (parsing ambiguity) and commas. null and leading/trailing whitespace remain rejected for both — input is Trim()-normalized before lookup, so allowing those would silently break round-tripping.
• Strings.resx: updated UnsupportedEnumIdentifier to describe the new rule ("It must not be null nor contain leading or trailing whitespace. Flags enums must additionally not be empty nor contain commas.").
• Tests:
  • EnumWithInvalidMemberName2 (empty identifier) is now marked [Flags] so it continues to validate the remaining rejection path.
  • Added EnumWithEmptyStringMemberName_NonFlags_RoundtripsAsExpected covering serialize/deserialize for "y", "n", and "" against the issue's YesOrNoOrEmpty enum.
  • Added EnumWithEmptyStringMemberName_Flags_Throws with a companion [Flags] YesOrNoOrEmptyFlags enum, asserting that the empty-string identifier is still rejected on both serialize and deserialize for flags enums.

Note

This pull request was created by an AI agent (GitHub Copilot Coding Agent) on behalf of the user.

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[eiriktsarpalis]

@copilot unless it already exists, please add test coverage for a similar flags enum.

[Copilot]

Added EnumWithEmptyStringMemberName_Flags_Throws with a YesOrNoOrEmptyFlags companion to YesOrNoOrEmpty, asserting that the empty-string identifier is still rejected for [Flags] enums on both serialize and deserialize. Commit 36a53ea.