More fixes for Razor multiline attribute formatting

[davidwengier]

Fixes dotnet/razor#13121 again

My previous fix didn't have enough testing, and then collection expressions came along and broke it in more ways. This time I've added a bunch more test coverage, most of which didn't need many changes, and also collection expression coverage/support, which needs specific handling lest Roslyn think a collection expression is an attribute list, due to the standalone way we write out C# expressions from attributes.

Some of the tests expected don't look that good, but I'm okay with it because a) it mirrors roslyn, which doesn't indent anything but the first line of a multiline expression in most cases and b) the results are at least stable, which is proven by multiple new tests, so worst case the user can manually fix up the results, and the formatter won't undo their work.

Microsoft Reviewers: Open in CodeFlow

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[Copilot]

Pull request overview

This PR adjusts Razor’s C# formatting-document generation so multiline attribute expressions—especially collection expressions—format deterministically and don’t “drift” indentation across repeated format passes. It also significantly expands regression coverage in the cohosting formatting test suite.

Changes:

• Updates the C# formatting document generator to account for collection-expression-specific parsing/indentation behaviors when formatting multiline attribute expressions.
• Adds collection-expression-aware scaffolding in start-tag/attribute handling to ensure Roslyn formats these expressions in a stable context.
• Adds extensive new unit tests covering stability and “unindented input → formatted output” scenarios across nesting and attribute-indent styles.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/Razor/src/Razor/test/Microsoft.CodeAnalysis.Razor.CohostingShared.UnitTests/Formatting/DocumentFormattingTest.cs	Adds/renames many formatting regression tests for multiline explicit expressions and collection expressions in attributes (stable + formatting scenarios).
src/Razor/src/Razor/src/Microsoft.CodeAnalysis.Razor.Workspaces/Formatting/Passes/CSharpFormattingPass.CSharpDocumentGenerator.cs	Updates the formatting-document generation logic to handle multiline collection expressions in attributes without indentation drift and with correct Roslyn parsing context.