Fix ArgumentException in SeparatedSyntaxList.ReplaceSeparator to use proper message and paramName (#80690)

Fix ArgumentException in SeparatedSyntaxList.ReplaceSeparator

Fixes the issue where `ReplaceSeparator` method throws
`ArgumentException` with parameter names as messages instead of proper
exception messages.

## Changes Made
- [x] Fix the two ArgumentException calls in ReplaceSeparator method to
use proper message and paramName parameters
- [x] Break combined RawKind/Language check into two separate checks
with dedicated resource strings
- [x] Build and test the changes
- [x] Code review

## Details
- Fixed first exception (separator not found) to use
`CodeAnalysisResources.MissingListItem` as message and
`nameof(separatorToken)` as paramName
- Split the combined RawKind/Language check into two separate checks:
- RawKind mismatch: Uses
`CodeAnalysisResources.SeparatorTokenMustHaveSameRawKind`
- Language mismatch: Uses
`CodeAnalysisResources.SeparatorTokenMustHaveSameLanguage`
- Added two new resource strings to CodeAnalysisResources.resx
- Updated .xlf files using dotnet msbuild /t:UpdateXlf
- All tests pass (10 SeparatedSyntaxListTests)

## Verification
Before fix:
- Exception Message: 'separatorToken', ParamName: ''
- Exception Message: 'newSeparator', ParamName: ''

After fix:
- Exception Message: 'The item specified is not the element of a list.
(Parameter 'separatorToken')', ParamName: 'separatorToken'
- Exception Message: 'New separator must have the same RawKind as the
separator being replaced. (Parameter 'newSeparator')', ParamName:
'newSeparator'
- Exception Message: 'New separator must have the same Language as the
separator being replaced. (Parameter 'newSeparator')', ParamName:
'newSeparator'

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<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *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](https://github.com/dotnet/roslyn/blob/8771786e5a80e9d27ffb40702f5cca558ca6fabb/src/Compilers/Core/Portable/Syntax/SeparatedSyntaxList.cs#L529)
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)
> 
> <comments>
> </comments>
> 


</details>

Fixes #2496

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---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: CyrusNajmabadi <4564579+CyrusNajmabadi@users.noreply.github.com>

Author: Copilot

src/Compilers/Core/Portable/CodeAnalysisResources.resx

@@ -438,6 +438,12 @@
   <data name="MissingListItem" xml:space="preserve">
     <value>The item specified is not the element of a list.</value>
   </data>
+  <data name="SeparatorTokenMustHaveSameRawKind" xml:space="preserve">
+    <value>New separator must have the same RawKind as the separator being replaced.</value>
+  </data>
+  <data name="SeparatorTokenMustHaveSameLanguage" xml:space="preserve">
+    <value>New separator must have the same Language as the separator being replaced.</value>
+  </data>
   <data name="InvalidPublicKey" xml:space="preserve">
     <value>Invalid public key.</value>
   </data>

src/Compilers/Core/Portable/Syntax/SeparatedSyntaxList.cs

@@ -573,13 +573,17 @@ public SeparatedSyntaxList<TNode> ReplaceSeparator(SyntaxToken separatorToken, S
             var index = nodesWithSeps.IndexOf(separatorToken);
             if (index < 0)
             {
-                throw new ArgumentException("separatorToken");
+                throw new ArgumentException(CodeAnalysisResources.MissingListItem, nameof(separatorToken));
             }
 
-            if (newSeparator.RawKind != nodesWithSeps[index].RawKind ||
-                newSeparator.Language != nodesWithSeps[index].Language)
+            if (newSeparator.RawKind != nodesWithSeps[index].RawKind)
             {
-                throw new ArgumentException("newSeparator");
+                throw new ArgumentException(CodeAnalysisResources.SeparatorTokenMustHaveSameRawKind, nameof(newSeparator));
+            }
+
+            if (newSeparator.Language != nodesWithSeps[index].Language)
+            {
+                throw new ArgumentException(CodeAnalysisResources.SeparatorTokenMustHaveSameLanguage, nameof(newSeparator));
             }
 
             return new SeparatedSyntaxList<TNode>(nodesWithSeps.Replace(separatorToken, newSeparator));