Plan: Add out-of-scope Blazor Component completions with auto-insert usings

• Address all code review feedback
• Remove TagHelperElementWithUsing enum value - use TagHelperElement instead
• Add AutoInsertNamespace property to RazorCompletionItem instead of parsing label
• Update RemoteCompletionService to use property instead of text parsing
• Simplify test implementation
• Store AdditionalTextEdits directly and populate during completion list creation
• Calculate AdditionalTextEdits in TagHelperCompletionProvider
• Remove AutoInsertNamespace property
• All tests passing

Implementation Summary

Final clean implementation:

1. RazorCompletionContext - Added CodeDocument parameter so providers have access to the document
2. TagHelperCompletionProvider - For fully qualified component names, calculates the AdditionalTextEdits directly using AddUsingsHelper and passes them to CreateTagHelperElement
3. RazorCompletionItem - Stores only AdditionalTextEdits property (no intermediate AutoInsertNamespace)
4. RemoteCompletionService - Simply reads AdditionalTextEdits property and applies them
5. RazorCompletionListProvider - No special processing needed, just passes CodeDocument through context

Architecture Benefits

This is the cleanest approach:

• All TextEdit generation happens at completion creation time in one place
• No intermediate properties or post-processing needed
• CodeDocument is available where needed through the context
• Simple and straightforward flow

Fixes #4634

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