Extract duplicated region location logic into shared helper function

[Copilot]

Addresses code duplication feedback from #48 where logic for computing region-end-line, git-relative-path, and region-location-info was duplicated between ai-code-discussion.el and ai-code-change.el.

Changes

• Added ai-code--get-region-location-info helper function in ai-code-discussion.el that returns (region-end-line git-relative-path region-location-info) given region boundaries
• Updated ai-code--ask-question-file and ai-code-code-change to use the helper instead of duplicating the logic
• Added forward declaration in ai-code-change.el

Before:

(region-end-line (when region-active
                   (line-number-at-pos (region-end))))
(git-relative-path (when (and region-active buffer-file-name)
                    (car (ai-code--get-git-relative-paths (list buffer-file-name)))))
(region-location-info (when (and region-active git-relative-path region-start-line region-end-line)
                       (format "%s#L%d-L%d" git-relative-path region-start-line region-end-line)))

After:

(region-info (when region-active
              (ai-code--get-region-location-info (region-beginning) (region-end))))
(region-end-line (nth 0 region-info))
(git-relative-path (nth 1 region-info))
(region-location-info (nth 2 region-info))

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