Add missing tests for resumeAt=2 path in regex expression conditional (#126657)

## Description

The Code Review workflow on PR #126561 identified that the new tests
cover `resumeAt=0` (yes-branch) and `resumeAt=1` (no-branch) paths but
miss the `resumeAt=2` pass-through path — expression conditional with
only a yes-branch (no no-branch) inside a loop.

Adds 3 test cases to `Regex.MultipleMatches.Tests.cs`:

- **Yes-only conditional, condition always fails** —
`(?((?'-1'))(?'1'.)+)+(?!(?'-1'))` with `"abc"` exercises the
pass-through path producing empty matches at each position (guarded with
`#if !NETFRAMEWORK` since .NET Framework produces 3 empty matches vs 4
on .NET Core due to different empty-match-at-end-of-string semantics)
- **Yes-only conditional with prefix capture, even input** —
`((?'1'.)(?((?'-1'))(?'1'.)))+` with `"abcd"` exercises the
yes-branch-taken path inside a loop
- **Yes-only conditional with prefix capture, odd input** — same pattern
with `"abc"` verifying partial match behavior

The latter two test cases are the primary coverage for the `|| isInLoop`
fix, since their non-backtracking yes-branch means `isInLoop` is the
necessary trigger for `resumeAt = 2` (the old condition
`postYesDoneLabel != originalDoneLabel` would have been false). These
run on both .NET Core and .NET Framework.

All 32,100 existing tests continue to pass.

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: danmoseley <6385855+danmoseley@users.noreply.github.com>
Co-authored-by: Dan Moseley <danmose@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 3 people

src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/Regex.MultipleMatches.Tests.cs

@@ -582,6 +582,22 @@ public static IEnumerable<object[]> Matches_TestData()
                             new CaptureData("d", 10, 1),
                         }
                     };
+
+                    // Yes-only expression conditional with prefix capture feeding the condition, inside a loop
+                    yield return new object[]
+                    {
+                        engine, @"((?'1'.)(?((?'-1'))(?'1'.)))+", "abcd", RegexOptions.None, new[]
+                        {
+                            new CaptureData("abcd", 0, 4),
+                        }
+                    };
+                    yield return new object[]
+                    {
+                        engine, @"((?'1'.)(?((?'-1'))(?'1'.)))+", "abc", RegexOptions.None, new[]
+                        {
+                            new CaptureData("ab", 0, 2),
+                        }
+                    };
                 }
 
 #if !NETFRAMEWORK // these tests currently fail on .NET Framework
@@ -611,6 +627,18 @@ public static IEnumerable<object[]> Matches_TestData()
                             new CaptureData("anyexpress1", 10, 11),
                         }
                     };
+
+                    // ExpressionConditional with only a yes-branch (no no-branch) inside a loop (empty matches differ on .NET Framework: https://github.com/dotnet/runtime/issues/24894)
+                    yield return new object[]
+                    {
+                        engine, @"(?((?'-1'))(?'1'.)+)+(?!(?'-1'))", "abc", RegexOptions.None, new[]
+                        {
+                            new CaptureData("", 0, 0),
+                            new CaptureData("", 1, 0),
+                            new CaptureData("", 2, 0),
+                            new CaptureData("", 3, 0),
+                        }
+                    };
                 }
 
                 // Fails on .NET Framework: https://github.com/dotnet/runtime/issues/62094