Add test coverage for refactored filepath candidate functions

[Copilot]

PR #155 removed three tests during refactoring but the underlying behaviors still needed verification. This PR restores test coverage adapted to the new implementation.

Tests Added

Helper function coverage:

• ai-code--normalize-path - path normalization for existing/non-existing files
• ai-code--candidate-path - relative (@-prefixed) paths for in-repo files, absolute paths for out-of-repo files
• ai-code--visible-window-files - file retrieval without git repo filtering (new behavior)
• ai-code--recent-buffer-paths - recent buffer collection with candidate formatting
• ai-code--current-frame-dired-paths - dired directory extraction

Integration behavior coverage:

• Visible files prioritized over buffer files in candidate list
• Dired directories included and prioritized in candidates
• Recent buffer paths include both files and directories

Example Test Structure

(ert-deftest ai-code-test-candidate-path-out-of-repo ()
  "Test that ai-code--candidate-path returns absolute path for out-of-repo files."
  (ai-code-with-test-repo
   (let ((out-file (expand-file-name "outside.el" temporary-file-directory)))
     (unwind-protect
         (progn
           (with-temp-file out-file (insert "content"))
           (let ((result (ai-code--candidate-path out-file (file-truename git-root))))
             ;; Should return absolute path (truename)
             (should (string= result (file-truename out-file)))))
       (when (file-exists-p out-file) (delete-file out-file))))))

Total: 9 test cases added covering all refactored functions and their integration behaviors.

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