fix: reject an inverted range when the comparer returns a negative other than -1

[dualfroz]

Problem

RangeValidator<T, TProperty> (used by InclusiveBetween / ExclusiveBetween) guards
against an inverted range in its constructor:

if(comparer.Compare(to,from)==-1){thrownewArgumentOutOfRangeException(nameof(to),"To should be larger than from.");}

The check compares the result against the literal -1. However, the
IComparer<T>.Compare / IComparable<T>.CompareTo contract only guarantees the sign
of the result: any value less than zero means "less than". It does not guarantee -1.

As a result the guard silently fails for legitimate comparers that return other negative
magnitudes:

• char bounds. char.CompareTo returns the arithmetic difference, e.g.
  'a'.CompareTo('c') == -2. So InclusiveBetween('c', 'a') (an inverted range) is
  accepted instead of throwing, producing a validator that can never pass.
• Custom IComparer<T> implementations passed to the InclusiveBetween /
  ExclusiveBetween overloads. Returning x - y (a very common pattern) yields
  magnitudes other than 1.
• Culture-sensitive string comparisons can also return magnitudes other than 1.

This is inconsistent with the existing behaviour for int (InclusiveBetween(10, 1)
throws, because int.CompareTo happens to return -1) and defeats the guard's stated
intent of rejecting to < from.

Fix

Compare against < 0 instead of == -1, matching the documented comparer contract.

src/FluentValidation/Validators/RangeValidator.cs

Tests

Added char-bound counterparts to the existing "to is smaller than from should throw"
tests in both InclusiveBetweenValidatorTests and ExclusiveBetweenValidatorTests.

Before the fix both new tests fail (no exception thrown); after the fix they pass. All
45 tests across the two between-validator suites pass, with no regressions.

Build / test gate

• SDK: .NET 10.0.400 (via global.json roll-forward), library TFM net8.0.
• dotnet build src/FluentValidation/FluentValidation.csproj -c Release -> 0 errors.
• dotnet test ... --filter "FullyQualifiedName~should_throw_for_chars":
  fails (2/2) before the fix, passes (2/2) after.
• dotnet test ... --filter "FullyQualifiedName~BetweenValidatorTests" -> 45 passed.