Throw ArgumentException for invalid DateTimeStyles in TryParse/TryParseExact

[OlegRa]

Summary

Closes #117.

DateOnly/TimeOnly TryParse and TryParseExact (span-based overloads) silently returned false for a DateTimeStyles value outside AllowWhiteSpaces, instead of throwing ArgumentException like Parse/ParseExact already do and like upstream dotnet/runtime's DateOnly/TimeOnly.TryParse now does.

• Added the same up-front (style & ~DateTimeStyles.AllowWhiteSpaces) != 0 check used by Parse/ParseExact to the three span-based Try* entry points on both types:
  • TryParse(ReadOnlySpan<char>, IFormatProvider?, DateTimeStyles, out T)
  • TryParseExact(ReadOnlySpan<char>, ReadOnlySpan<char>, IFormatProvider?, DateTimeStyles, out T)
  • TryParseExact(ReadOnlySpan<char>, string?[]?, IFormatProvider?, DateTimeStyles, out T)
• The string-based overloads delegate into these, so they pick up the fix automatically once their s/format/formats null checks pass.
• Scope intentionally limited to the DateTimeStyles validation gap (TryParse/TryParseExact should throw for invalid DateTimeStyles, not return false #117). The separate formats[] up-front validation gap (ParseExact/TryParseExact(formats[]) should validate the whole array up front #118) and the malformed-format-string gap (TryParseExact silently returns false for a malformed custom format string instead of throwing #119) are not touched here.

Tests

• Fixed two pre-existing tests in DateOnlyTests.cs/TimeOnlyTests.cs (BasicFormatParseTest) that literally asserted the old buggy TryParse behavior (Assert.False(...)) right next to a correct Parse assertion (Assert.Throws<ArgumentException>(...)) for the same invalid style — now both assert the throw.
• Updated ArgumentValidationTests.cs: two existing TryParseExact assertions that used an invalid style (AssumeUniversal/AssumeLocal) with non-null input now expect ArgumentException instead of false.
• Added new coverage for the TryParse(span/string) and TryParseExact(formats[]) overloads with an invalid style, since none of those specific paths had a non-null-input test before.

Test plan

• dotnet build -c Debug — succeeds, 0 warnings/errors
• dotnet test — 169/169 passed

[Copilot]

Pull request overview

This PR aligns DateOnly/TimeOnly span-based TryParse/TryParseExact behavior with Parse/ParseExact (and upstream dotnet/runtime) by throwing ArgumentException when DateTimeStyles contains unsupported flags, instead of silently returning false.

Changes:

• Added upfront (style & ~DateTimeStyles.AllowWhiteSpaces) != 0 validation (throwing ArgumentException) to the span-based TryParse/TryParseExact entry points for both DateOnly and TimeOnly.
• Updated/expanded tests to expect ArgumentException for invalid styles in the affected Try* overloads and added coverage for previously untested paths.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
DateTimeOnly/TimeOnly.cs	Adds upfront invalid-DateTimeStyles checks to span-based TryParse/TryParseExact overloads so invalid styles throw consistently.
DateTimeOnly/DateOnly.cs	Same upfront invalid-DateTimeStyles checks for DateOnly span-based TryParse/TryParseExact overloads.
DateTimeOnly.Tests/TimeOnlyTests.cs	Updates a pre-existing test to assert invalid styles throw for TimeOnly.TryParse (matching Parse).
DateTimeOnly.Tests/DateOnlyTests.cs	Updates a pre-existing test to assert invalid styles throw for DateOnly.TryParse (matching Parse).
DateTimeOnly.Tests/ArgumentValidationTests.cs	Updates existing assertions to expect ArgumentException (not false) and adds new coverage for invalid-style TryParse/TryParseExact paths.

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