Fix verbatim escaping in regex code generator

[PranavSenthilnathan]

When the pattern expression is folded into a constant that needs escaping, we choose to use a verbatim string literal. However, verbatim strings also need to escape one character - the double quote.

The code generated for

var isMatch = Regex.IsMatch("", "[" + @"\/:<>|" + "\"]");

is now:

[GeneratedRegex(@"[\/:<>|""]")]
private static partial Regex MyRegex();

Fixes #104371

[Copilot]

Pull Request Overview

This PR fixes the verbatim escaping of double quotes in the regex code generator.

• Adds a new test to verify correct escaping when constant pattern expressions include quotes.
• Updates the code fixer to replace double quotes with two double quotes for verbatim string literals.

Reviewed Changes

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

File	Description
src/libraries/System.Text.RegularExpressions/tests/FunctionalTests/UpgradeToGeneratedRegexAnalyzerTests.cs	Added a new test case to validate escaping of quotes in regex patterns.
src/libraries/System.Text.RegularExpressions/gen/UpgradeToGeneratedRegexCodeFixer.cs	Modified the code fixer to correctly escape double quotes in verbatim strings.

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