Fix toLocaleString timeZoneName returning full name instead of abbreviation

[Copilot]

toLocaleString with timeZoneName: 'short' was returning the full .NET timezone name (e.g., "Eastern Standard Time") instead of the proper IANA abbreviation ("EST"/"EDT"). The same issue affected timeStyle: 'full'/'long' and timeZoneName: 'long', which always returned the standard name regardless of DST.

Root Cause

GetTimeZoneDisplayName in JsDateTimeFormat had a broken fallback for the no-explicit-timezone case:

// Before: always standard name, always TimeZoneInfo.Local, never DST-aware
return longName ? TimeZoneInfo.Local.StandardName : TimeZoneInfo.Local.Id;

Three bugs in one line:

• Used TimeZoneInfo.Local instead of the engine's configured default timezone
• No DST check — always returned StandardName even during daylight saving time
• Short name returned the Windows timezone ID ("Eastern Standard Time"), not an abbreviation

Fix

Replaced the fallback with proper logic mirroring the explicit-timezone path:

var defaultTz = _engine.Options.TimeSystem.DefaultTimeZone;
// DST check (consistent with TimeZone != null path)
defaultIsDst = defaultTz.IsDaylightSavingTime(DateTime.SpecifyKind(utcDateTime, DateTimeKind.Utc));

// Convert Windows ID → IANA for CLDR lookup (e.g. "Eastern Standard Time" → "America/New_York")
#if NET6_0_OR_GREATER
if (!defaultTz.HasIanaId && TimeZoneInfo.TryConvertWindowsIdToIanaId(defaultTzId, out var defaultIanaId))
    defaultTzId = defaultIanaId;
#endif

// CLDR metazone data returns "EDT"/"EST", "Eastern Daylight Time"/"Eastern Standard Time", etc.
var defaultCldrName = Data.MetaZoneData.GetDisplayName(defaultTzId, defaultIsDst, longName, generic);

Result:

new Date(2022, 6, 4).toLocaleString('en-US', { timeZoneName: 'short' })
// Before: "7/4/2022, 12:00:00 AM Eastern Standard Time"
// After:  "7/4/2022, 12:00:00 AM EDT"

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[lahma]

@copilot check windows ci failure

[Copilot]

@copilot check windows ci failure

The Windows CI failure was in Jint.Tests.exe (net472): ToLocaleStringUsesShortTimeZoneAbbreviation failed because TimeZoneInfo.TryConvertWindowsIdToIanaId isn't available on .NET Framework 4.7.2 (it's wrapped in #if NET6_0_OR_GREATER). Without it, the Windows timezone ID "Eastern Standard Time" wasn't converted to "America/New_York" before the CLDR lookup, so the lookup failed and the fallback returned the wrong value.

Fixed in a13db30 by adding Windows timezone ID aliases directly to the IanaToMetaZone dictionary in MetaZoneData.cs (covering ~70 common Windows timezone IDs). This makes the CLDR lookup work on all .NET versions. Also fixed the final fallback for short names to be DST-aware when the timezone ID has no '/' (as Windows IDs do).