Fix MA0002 code fix inserting StringComparer at wrong argument position

[meziantou]

Fixes #1249 and #1250.

Problem

The UseStringComparerFixer (MA0002) was always appending the StringComparer argument at the end of the argument list. This broke the build when the target overload has the comparer parameter before other parameters:

• MA0002 autofix breaks the build on Assert.AreEqual with 3 parameters #1249: Assert.AreEqual(expected, actual, message) → incorrectly became AreEqual(expected, actual, message, StringComparer.Ordinal) instead of AreEqual(expected, actual, StringComparer.Ordinal, message)
• MA0002 autofix breaks the build on EntityFrameworkQueryableExtensions.ToDictionnaryAsync with cancellationtoken #1250: Extension method with comparer before CancellationToken → same issue

Fix

The fix follows the existing UseIFormatProviderFixer pattern:

1. Find the target overload using OverloadFinder.FindOverloadWithAdditionalParameterOfType
2. Compute insertion index via a new TryGetComparerParameterInfo helper that:
   • Strips the implicit this parameter from extension methods (so the index aligns with the actual argument list, not the full parameter list)
   • Returns both the index and the parameter name (for named-argument fallback)
3. Insert positionally when index <= currentArgCount, or use a named argument when the comparer falls beyond the current args (handles optional-params-before-comparer edge case)

The key insight for #1250 (extension methods): IInvocationOperation.TargetMethod returns the non-reduced extension method (including this in its parameters), but the argument list doesn't include the receiver. The new GetComparableParameters helper always strips this from extension methods regardless of form (reduced or not), ensuring the insertion index maps correctly to the argument list position.