Fix escaping of UNC project paths containing spaces

[innwayyy]

Fixes #38675

dotnet-ef incorrectly escaped backslashes in UNC project paths when generating the dotnet msbuild argument list. As a result, paths starting with \\FILESERVER were passed as \\\\FILESERVER.

This change preserves the UNC prefix while continuing to quote paths containing spaces.

A regression test was added that fails before the fix and passes after it.

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Summary of the changes

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Fixes #bugnumber

• Tests for the changes have been added (for bug fixes / features)
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[innwayyy]

@dotnet-policy-service agree

[Copilot]

Pull request overview

Fixes incorrect escaping of backslashes in dotnet-ef argument-string generation for quoted (space-containing) UNC paths, ensuring the leading \\SERVER prefix is preserved when invoking dotnet msbuild.

Changes:

• Adjusted Exe.ToArguments to emit pending backslashes verbatim when they are not escaping a " or the closing quote.
• Updated the existing argument-escaping unit test expectation to match correct backslash handling.
• Added a regression test covering UNC paths with spaces to prevent reintroducing the \\\\SERVER behavior.

Reviewed changes

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

File	Description
src/dotnet-ef/Exe.cs	Fixes backslash emission logic so quoted UNC paths keep the \\ prefix (no unintended doubling).
test/dotnet-ef.Tests/ExeTest.cs	Updates existing expectation and adds a targeted regression test for UNC-prefix handling.