Harden FileSystem init error handling and align file-exists test constants

[Copilot]

This PR addresses the review-thread feedback on FileSystem initialization and its regression coverage. It removes broad error suppression during cache directory setup and aligns tests with the shared file-exists error constants used by production code.

• Initializer error handling

  • Replaced fail-fast try! directory creation with explicit do/catch handling.
  • Treats only NSError.isFileExistsError as ignorable; other creation failures are surfaced via assertionFailure(...).
  • Keeps directory creation idempotent by using withIntermediateDirectories: true.

• Targeted regression tests

  • Added FileSystemInitializationTests for:
    • init behavior when createDirectory returns “file exists”
    • repeated init when parent directory already exists
  • Updated the file-exists test double to use NSError.FileErrors.domain and NSError.FileErrors.fileExistsErrorCode (no duplicated raw 516).

do {
    try backingManager.createDirectory(at: self.parentDirectory, withIntermediateDirectories: true, attributes: nil)
} catch {
    if !(error as NSError).isFileExistsError {
        assertionFailure("Failed to create cache directory at \(self.parentDirectory.path): \(error)")
    }
}