Reduce code duplication in converters and event emitters

[fdcastel]

Summary

Reduce code duplication across DateTime/TimeOnly converters and event emitter scalar rendering.

Extract ScalarConverterBase<T> for converter family

Created new abstract base class ScalarConverterBase<T> providing:

• Generic Accepts(Type) method — eliminates identical boilerplate from 5 converter classes
• ConsumeScalarValue() helper for reading scalar values from the parser
• EmitScalar() helper for writing scalars with a given style

Migrated 5 converters to inherit from the base:

• DateTimeConverter
• DateTimeOffsetConverter
• DateTime8601Converter
• DateOnlyConverter
• TimeOnlyConverter

Align event emitter scalar rendering

• JsonEventEmitter: Use formatter.FormatNumber() for float, double, and decimal instead of direct CultureInfo.InvariantCulture formatting. This aligns with TypeAssigningEventEmitter and ensures the configured formatter is respected. NaN/Infinity double-quoting is preserved.
• TypeAssigningEventEmitter: Use .IsEnum() extension method instead of .IsEnum property for enum detection, matching the JsonEventEmitter pattern.
• Removed unused System.Globalization and System.Text.RegularExpressions imports from JsonEventEmitter.

Files Changed

• YamlDotNet/Serialization/Converters/ScalarConverterBase.cs (new)
• YamlDotNet/Serialization/Converters/DateTimeConverter.cs
• YamlDotNet/Serialization/Converters/DateTimeOffsetConverter.cs
• YamlDotNet/Serialization/Converters/DateTime8601Converter.cs
• YamlDotNet/Serialization/Converters/DateOnlyConverter.cs
• YamlDotNet/Serialization/Converters/TimeOnlyConverter.cs
• YamlDotNet/Serialization/EventEmitters/JsonEventEmitter.cs
• YamlDotNet/Serialization/EventEmitters/TypeAssigningEventEmitter.cs

Validation

All 1845 tests pass (4 pre-existing locale-dependent float formatting failures unrelated to these changes).

[fdcastel]

Fixed the CI failure in commit 07c17d4.

Root cause: The refactoring of JsonEventEmitter to use formatter.FormatNumber() instead of direct CultureInfo.InvariantCulture formatting inadvertently changed how NaN and Infinity values are rendered. The YamlFormatter.FormatNumber() uses YAML-style symbols (.nan, .inf, -.inf), but JSON output needs the standard .NET representations (NaN, Infinity, -Infinity).

Fix: For float/double values that are NaN or Infinity, use CultureInfo.InvariantCulture formatting (preserving original behavior). For regular numeric values, continue using formatter.FormatNumber() (the deduplication goal of this PR).

The SerializationOfNumericsAsJsonRountTrip test now passes on all frameworks.