Add index to OrderedDictionary debugger display

[Copilot]

Summary

This PR adds index information to the debugger display of OrderedDictionary<TKey, TValue> to show the position of each element in the ordered collection. The debugger display format is now [INDEX/KEY, VALUE] instead of just [KEY, VALUE].

Problem

Since .NET 9, the debugger display for dictionaries changed from [INDEX, KEY/VALUE] to [KEY, VALUE], using the generic IDictionaryDebugView. While this works well for standard dictionaries where order is not significant, OrderedDictionary maintains insertion order and supports index-based access as a key feature. Losing the index information in the debugger makes it difficult to understand the element positions and order.

Solution

Created a custom OrderedDictionaryDebugView<TKey, TValue> debug proxy that displays items with both index and key information:

Before: [10, "Ten"], [20, "Twenty"], [30, "Thirty"]
After:  [0/10, "Ten"], [1/20, "Twenty"], [2/30, "Thirty"]

The new format clearly shows:

• Index (0, 1, 2): The position in the ordered collection
• Key (10, 20, 30): The dictionary key for lookup
• Value: The associated value

Changes

1. OrderedDictionaryDebugView.cs: New custom debugger type proxy with DebugViewOrderedDictionaryItem<TKey, TValue> struct using [DebuggerDisplay("{Value}", Name = "[{Index}/{Key}]")]

2. OrderedDictionary.cs: Updated DebuggerTypeProxy attribute from IDictionaryDebugView<,> to OrderedDictionaryDebugView<,>

3. System.Collections.csproj: Added new file to compilation

4. DebugView.Tests.cs: Added test cases validating the [INDEX/KEY] format for both empty and populated OrderedDictionaries

Testing

• ✅ All 3,781 existing OrderedDictionary tests pass
• ✅ All 108 DebugView tests pass (including new test cases)
• ✅ CodeQL security analysis: No issues found

Fixes #issue_number

Original prompt

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This section details on the original issue you should resolve

<issue_title>Add index to debugging display of OrderedDictionary<TKey, TValue></issue_title>
<issue_description>Since .NET 9 the new display info for dictionaries is [KEY, VALUE] coming from previous [INDEX, KEY/VALUE].

But index information is important here, so please add index => [INDEX/KEY, VALUE].</issue_description>

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

Fixes #118590

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