Fix relational operators failing for nullable IComparable types (e.g., Instant?)

[Copilot]

Relational operators (>, >=, <, <=) threw Operator '>' incompatible with operand types 'Instant?' and 'Instant?' for any nullable struct type implementing IComparable<T> that isn't explicitly listed in IRelationalSignatures.

Root cause

Nullable<T> does not itself implement IComparable<T> — only the underlying T does. In ParseComparisonOperator, the interface check was called directly on the nullable type, so typesAreSameAndImplementCorrectInterface was false for Instant?, falling through to CheckAndPromoteOperands which had no matching signature.

Fix

Use TypeHelper.GetNonNullableType() before the interface check so that T? is treated the same as T for IComparable<> and IEquatable<> resolution:

// Before
var interfaces = left.Type.GetInterfaces().Where(x => x.GetTypeInfo().IsGenericType);

// After
var typeToCheck = TypeHelper.GetNonNullableType(left.Type);
var interfaces = typeToCheck.GetInterfaces().Where(x => x.GetTypeInfo().IsGenericType);

This applies to both the IComparable<> check (relational operators) and the IEquatable<> check (equality operators) for consistency.

Tests

Added parameterized tests in NodaTimeConverterTests covering all relational operators (>, >=, <, <=) as well as equality operators (==, !=) against both Instant and Instant? properties, with count assertions verifying correct filtering semantics (including that null entries satisfy null != value in C# nullable semantics for the nullable case).

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