T4 follow-up: negative-int + MinValue/MaxValue edge cases

[Chris-Wolfgang]

Summary

Follow-up to PR #134 (T4 test-quality audit). Closes the smaller
edge-case gaps the audit noted but skipped at the time — the bulk
(custom IComparable<T> + null-bound contracts) landed in #134;
this PR is purely additive coverage.

Stacked on #134

Base = stack/t4-test-quality-audit. When #134 merges, this PR's
base auto-promotes to vNext.

What this adds

1. Negative integer range Theories (10 rows)

Every existing int Theory used positive bounds (1, 10). Added a
parallel Theory with bounds (-10, 0) covering below, exact-lower,
mid, exact-upper, above for both methods. By construction these
exercise the same code path as the positive cases, but a future
refactor introducing an Abs-based shortcut or an unsigned compare
would now break loudly.

2. int.MinValue / int.MaxValue boundary Facts (4)

Four Facts covering value = int.MaxValue and value = int.MinValue
against (int.MinValue, int.MaxValue) bounds for both methods.
Confirms strict bounds reject the exact extremes (IsBetween → false)
and inclusive bounds accept them (IsInRange → true).

3. Money record documentation (comment only)

The Money fixture introduced in #134 stipulates that
CompareTo(null) returns +1 (null is the smallest value). That's a
valid convention per IComparable guidance but the choice was
hidden in the record definition. Added a comment explaining the
rationale.

Skipped from the audit notes — not worth adding

• Inverted bounds for non-int types (gap Initial development #1 from the followup
  list): behaviour is type-agnostic, adding string/DateTime/char rows
  would be padding.
• Equal-bounds for non-int types (Bump version to 1.0.0 for stable release #2): same reason.
• String edge cases ("", very long strings) (Change license from Mozilla Public to MIT #5): lexicographic
  comparison is well-defined; gain is small.
• Redundant DateTime Theory (Bump Microsoft.NET.Test.Sdk from 17.11.1 to 18.0.1 #6): both the string-typed parser
  and the typed-DateTime variant exercise the same behaviour; removing
  either loses nothing but adds churn.
• Double-CompareTo invocation testability (Add comprehensive documentation files for IComparable Extensions #8): not testable
  without a mock, not a real risk.

Verified

$ dotnet test ... -c Release --framework net10.0
Passed!  Failed: 0, Passed: 72, Skipped: 0, Total: 72

72 cases — 58 after #134 + 10 negative-int rows + 4 MinValue/MaxValue
Facts.

[Copilot]

Pull request overview

Adds incremental unit-test coverage for IsBetween (exclusive) and IsInRange (inclusive) to lock in behavior for negative integer ranges and int.MinValue/int.MaxValue, plus a clarifying comment on the Money test fixture’s CompareTo(null) convention.

Changes:

• Add negative-bound integer [Theory] coverage for both IsBetween and IsInRange.
• Add Facts covering int.MinValue/int.MaxValue against full-range bounds for both methods.
• Document the Money.CompareTo(null) convention used in custom IComparable<T> tests.