Add test coverage for DateTime.ParseExact with different DateTimeStyles

[Copilot]

Description

Improves test coverage for DateTime.ParseExact with DateTimeStyles values that had minimal or no coverage, per corefx#27678.

New test cases added to ParseExact_ValidInput_Succeeds_MemberData:

• RoundtripKind (3 cases) — verifies UTC kind preservation with 'K' format specifier
• AssumeLocal (2 cases) — verifies dates without timezone info are interpreted as local
• NoCurrentDateDefault (2 cases) — verifies 0001-01-01 is used when date components are missing
• AllowInnerWhite standalone (2 cases) — verifies inner whitespace handling

Expanded InvalidDateTimeStyles test:

Converted from [Fact] to [Theory] with invalid style combinations:

[InlineData(DateTimeStyles.AssumeLocal | DateTimeStyles.AssumeUniversal)]
[InlineData(DateTimeStyles.RoundtripKind | DateTimeStyles.AssumeLocal)]
[InlineData(DateTimeStyles.RoundtripKind | DateTimeStyles.AssumeUniversal)]
[InlineData(DateTimeStyles.RoundtripKind | DateTimeStyles.AdjustToUniversal)]

All 1356 DateTime tests pass.

Original prompt

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This section details on the original issue you should resolve

<issue_title>Improve test coverage of DateTime ParseExact with different DateTimeStyles</issue_title>
<issue_description>See dotnet/corefx#27678 for context.

@joperezr, @AlexGhiondea, @stephentoub, @jkotas </issue_description>

<agent_instructions>Only add tests for features that aren't already appropriately covered. Try to augment existing theories wherever possible and add to existing test files wherever possible. Follow the same style as the rest of the file being augmented. </agent_instructions>

Comments on the Issue (you are @copilot in this section)

@ViktorHofer @AlexGhiondea As ParseExact is nothing new I believe this can safely be switched to future? @joperezr @ViktorHofer agreed 😄

• Fixes Improve test coverage of DateTime ParseExact with different DateTimeStyles #25295

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[Copilot]

Pull request overview

This pull request enhances test coverage for DateTime.ParseExact with various DateTimeStyles values that previously had minimal or no coverage, addressing issue #25295 (originally corefx#27678).

Changes:

• Converted InvalidDateTimeStyles test from a single [Fact] to a parameterized [Theory] with 4 test cases covering invalid style combinations
• Added 9 new test cases to ParseExact_ValidInput_Succeeds_MemberData covering RoundtripKind (3), AssumeLocal (2), NoCurrentDateDefault (2), and AllowInnerWhite (2)

[stephentoub]

/ba-g timeouts