Fix TODOs, typos, and add missing tests

[fdcastel]

Summary

This PR resolves stale TODO comments, fixes typos in user-facing messages, and adds missing test coverage across three areas.

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1. Fix typos in error and documentation messages

Multiple user-facing error messages and XML documentation comments contain typos:

File	Typo Fixed
YamlDotNet/Serialization/IObjectDescriptor.cs	whete → where
YamlDotNet/Serialization/DeserializerBuilder.cs	maped → mapped (×2)
YamlDotNet/Serialization/SerializerBuilder.cs	maped → mapped
YamlDotNet/Serialization/StaticDeserializerBuilder.cs	maped → mapped (×2)
YamlDotNet/Serialization/StaticSerializerBuilder.cs	maped → mapped
YamlDotNet/Serialization/Converters/DateTimeConverter.cs	serializer complext → serialize complex
YamlDotNet/Serialization/Converters/DateTimeOffsetConverter.cs	serializer complext → serialize complex
YamlDotNet/Serialization/Converters/DateTime8601Converter.cs	serializer complext → serialize complex
YamlDotNet/Serialization/Converters/TimeOnlyConverter.cs	serializer complext → serialize complex

2. Resolve TODO comments in ScalarNodeDeserializer

ScalarNodeDeserializer.cs contained three TODO comments marking incomplete or questionable logic:

1. DateTime parsing TODO: Replaced misleading "probably incorrect" comment with a clarifying note — DateTime.Parse with InvariantCulture and RoundtripKind is correct when the target type is explicitly DateTime.
2. Binary/octal numberFormat TODO: Replaced with a comment explaining that binary/octal parsing is inherently locale-independent; Convert.ToUInt64 does not accept an IFormatProvider for these bases.
3. Base-60 validation TODO: Implemented actual digit validation — chunks after the first in sexagesimal notation must be in range [0, 59]. Values like 1:99 now throw a FormatException.

Tests added: DateTime_ParsesValidValues, DateTime_ThrowsOnInvalidValues, Base60Integer_ParsesValidValues, Base60Integer_RejectsInvalidSexagesimalDigit, Base60Integer_AllowsFirstChunkAbove59, Base60Integer_RejectsSecondChunkOf60, IntegerBases_ParseCorrectly

3. Remove stale TODO comments from required-properties enforcement

ObjectNodeDeserializer.cs contained two stale TODO comments. The implementation was already complete (checks property.Required and throws YamlException listing missing property names). Removed the leftover markers.

Tests added: WithRequiredMemberSet_ThrowsWhenBothMissing_ListsBothNames, WithRequiredMemberSet_ThrowsWithDescriptiveMessage, WithoutEnforceRequiredMembers_DoesNotThrowWhenMissing

[fdcastel]

The CI was failing due to a conflict between the new sexagesimal validation added in this PR and a pre-existing test.

Root cause: ScalarNodeDeserializer.DeserializeIntegerHelper was updated to enforce that sexagesimal (base-60) digits after the first must be in the range [0, 59]. The existing DeserializeScalarLongBase60Number test used the value "99_:_58:47:3:6_2:10", where 6_2 (= 62) is not a valid base-60 digit, so the new validation correctly rejected it.

Fix: Changed the test input to "99_:_58:47:3:5_2:10" (using 5_2 = 52, which is a valid base-60 digit) and updated the expected result from 77744246530L to 77744245930L.