Handle Windows backslashes in ConfigurationUpdater header regex construction

[Copilot]

ConfigurationUpdater was building a regex directly from .editorconfig header text and did not handle Windows-style \ path separators. In VS Code quick-fix flows (for analyzer severity configuration), this could throw RegexParseException (for example, \h escape) instead of applying/updating configuration.

• Product fix: normalize header path separators before regex translation

  • In ConfigurationUpdater.CheckIfRuleExistsAndReplaceInFile, header path text now normalizes \ to / before wildcard/extension regex expansion.
  • This avoids invalid escape sequences while preserving existing matching behavior against normalized relative paths.

• Regression coverage: backslash-heavy header scenario

  • Added a focused test in CategoryBasedSeverityConfigurationTests to cover a header containing Windows-style backslashes.
  • The test ensures configuration update proceeds through the code action path that previously failed with invalid regex parsing.

// Before translating header text into regex fragments:
fileName = fileName.Replace('\\', '/');
fileName = fileName.Replace(".", @"\.");
fileName = fileName.Replace("*", ".*");