Fix MA0184 code fix keeping doubled braces as literal text

[meziantou]

Problem

The MA0184 code fix ("Convert to regular string") copied InterpolatedStringTextSyntax.TextToken.ValueText into a regular string literal. For non-raw interpolated strings, that text still contains the brace escapes ({{ and }}), which a regular string literal does not interpret, so the fix changed the value of the string:

public static object Run() => $"{{text}}"; // returns {text}
// after the fix
public static object Run() => "{{text}}";  // returns {{text}}

Both versions compile, so JSON fragments and other brace-containing text were silently corrupted.

Fix

For non-raw interpolated strings, the fixer now builds the literal from the value computed by the compiler: the constant values of the IInterpolatedStringTextOperation parts, which have the braces unescaped. The replacement is computed in RegisterCodeFixesAsync, and the fix is not registered when the value cannot be obtained. The raw string path is unchanged.

Tests

Added to DoNotUseInterpolatedStringWithoutParametersAnalyzerTests:

• $"{{text}}" → "{text}"
• $@"{{""text"": ""\""}}" → "{\"text\": \"\\\"}" (verbatim string with escaped braces, quotes, and a backslash)
• $"" → ""

The two brace tests fail with the previous fixer. The test class passes (16/16) on Roslyn 4.8, 4.14, 5.0, 5.6 and 5.9. dotnet run --project src/DocumentationGenerator produces no changes.

Notes for reviewers

• Not addressed here: the raw string path removes only the first $, so $$"""{text}""" becomes $"""{text}""", which turns {text} into an interpolation. Testing it is awkward because the testing library's markup parser treats $$ as the caret position marker, so I left it for a separate change.
• Verbatim interpolated strings are still converted to regular (escaped) literals rather than verbatim ones, as before.