Support ISO 8601 24:00:00 (endOfDayFrag) in XSD DateTime validation

[Copilot]

Description

Fixes #29005

XML Schema validation rejected 24:00:00 (endOfDayFrag) even though W3C XML Schema 1.1 Part 2 explicitly defines it as valid. PR #124142 added the underlying ISO 8601 24:00 support to DateTime/DateTimeOffset/TimeOnly parsing; this extends that support to the XSD layer used by XmlConvert and XML Schema validation.

Changes

• XsdDateTime.ParseTime(): hour < 24 → hour <= 24, with post-parse validation rejecting hour=24 when minute, second, or fraction are non-zero
• XsdDateTime.InitiateXsdDateTime() → TryInitiateXsdDateTime(): When hour=24, sets hour=0 and adds one day. Returns false on overflow (e.g., 9999-12-31T24:00:00) instead of throwing
• TryParse() / constructor: Updated to use TryInitiateXsdDateTime for exception-free overflow handling
• 47 new test cases: Valid/invalid endOfDay across xs:dateTime, xs:time, XDR variants; schema validation end-to-end; leap year/year boundary transitions; fractional seconds; overflow; regression coverage for hours 0–23

// Previously threw FormatException; now parses correctly
var dt = XmlConvert.ToDateTime("2007-04-05T24:00:00", XmlDateTimeSerializationMode.RoundtripKind);
// dt == 2007-04-06T00:00:00 (next day's midnight, per spec)

// Schema validation now accepts endOfDayFrag
var settings = new XmlReaderSettings { ValidationType = ValidationType.Schema };
settings.Schemas.Add("ns", XmlReader.Create(new StringReader(xsd)));
using var reader = XmlReader.Create(new StringReader(xml), settings);
new XmlDocument().Load(reader); // no longer throws

Breaking change considerations

This is a relaxation — previously rejected inputs now parse successfully:

• 2007-04-05T24:00:00 → 2007-04-06T00:00:00 (was FormatException)
• 24:00:00 as xs:time → valid (was validation error)
• 24:01:00, 24:00:01, 24:00:00.0000001 remain invalid
• 9999-12-31T24:00:00 remains invalid (DateTime overflow)
• Round-trip: serializing the parsed result produces 00:00:00 on the next day, not the original 24:00:00 input — consistent with ISO 8601 semantic equivalence

Code relying on FormatException for 24:00:00 inputs or on schema validation rejecting them will observe changed behavior.

Adjacent scope

All XSD types with time components share the same ParseTime() path: xs:dateTime, xs:time, and XDR datetime/time variants are all covered. Types without time components (xs:date, xs:gYear, etc.) are unaffected.

Warning

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• bla
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• foo
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• test.test
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[jeffhandley]

@tarekgh Tagged you for review next week. Not urgent.