Fix ArgumentException in SeparatedSyntaxList.ReplaceSeparator to use proper message and paramName

[Copilot]

Problem

The ReplaceSeparator method in SeparatedSyntaxList<TNode> was incorrectly throwing ArgumentException by passing parameter names as exception messages instead of using the proper constructor overload. This resulted in exceptions with confusing single-word messages like "separatorToken" or "newSeparator" and empty ParamName properties.

// Before - incorrect usage
throw new ArgumentException("separatorToken");  // Message: "separatorToken", ParamName: ""
throw new ArgumentException("newSeparator");    // Message: "newSeparator", ParamName: ""

Solution

Updated both ArgumentException throws to use the correct constructor that accepts both a message and a parameter name:

1. When separator token is not found in the list:

   • Now uses CodeAnalysisResources.MissingListItem resource string with nameof(separatorToken)
   • Message: "The item specified is not the element of a list."
   • ParamName: "separatorToken"

2. When new separator has incompatible RawKind or Language:

   • Now uses a descriptive inline message with nameof(newSeparator)
   • Message: "New separator must have the same RawKind and Language as the separator being replaced."
   • ParamName: "newSeparator"

// After - correct usage
throw new ArgumentException(CodeAnalysisResources.MissingListItem, nameof(separatorToken));
throw new ArgumentException("New separator must have the same RawKind and Language as the separator being replaced.", nameof(newSeparator));

Fixes #

Original prompt

---

This section details on the original issue you should resolve

<issue_title>SeparatedSyntaxList.ReplaceSeparator throws ArgumentException with bad message</issue_title>
<issue_description>The ReplaceSeparator method throws ArgumentException passing in the parameter name instead of a message as required by the exception constructor. This results in exceptions with odd one-word messages being thrown.
</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #2496

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[CyrusNajmabadi]

@copilot remove these tests

[Copilot]

Done. Removed the test methods in commit 7c89d.