feat(editors): let image blocks switch between URL and Slack file sources

[StephenTangCook]

Summary

Adds an "Image source" radio (Public URL / Slack file) to the image block editor flyout, so a slack_file-backed image block can be converted to the image_url variant (and back), instead of rendering as a dead-end "not editable" message. The slack_file reference itself (File ID / File URL) plus alt text and title are now editable in both variants.

Why

The builder doesn't support slack_file (no upload flow, and the preview can't render workspace files), so an image block loaded from an existing message with a slack_file backing was completely stuck — the flyout showed "Not editable in the visual builder" with no way to switch it to a URL-backed image.

Implementation notes:

• Slack rejects an image block carrying both image_url and slack_file (verified against @tightknitai/slack-block-kit-validator), so the source switch swaps one backing field for the other. The replaced default is spread first so a malformed block that already carries both keys keeps its existing value.
• Switching to Public URL intentionally starts from an empty image_url: the flyout's validation error nudges the user to paste a real URL rather than silently sending a placeholder image.
• Within slack_file, Slack accepts {id} or {url} (not both), so editing one field rewrites the reference and clears the other.
• The radio option is labeled "Public URL" (not "Image URL") so it doesn't collide with the Image URL input's accessible label.
• The new SwitchingImageSourceProducesValidBlock story overrides the meta useArgs render with local state: updateArgs deep-merges args, and a merge can't express removing the slack_file key.

Test plan

• pnpm typecheck
• pnpm lint
• pnpm test — 513 passed (44 files), including 8 new unit tests in test/image-editor.test.tsx and 2 new stories (a static slack_file editor story and a browser play test converting slack_file → URL end-to-end)
• pnpm build and pnpm build-storybook

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