IMAGES-2191: improve variant URLs returned by miniflare hosted images mock

[mglewis]

The miniflare hosted images mock currently returns bare variant names (e.g. "public") in the variants field of ImageMetadata. In production, this field contains full delivery URLs such as https://imagedelivery.net/<account_hash>/<image_id>/public. These bare names are not usable in an <img src=...>, resulting in variant URLs failing to display when running in local mode.

This PR introduces a local image delivery endpoint at /cdn-cgi/imagedelivery/<image_id>/<variant> that serves image bytes directly from the existing miniflare KV store. The variants field now returns relative paths pointing to this endpoint (/cdn-cgi/imagedelivery/<image>/public), making them functional in a browser during local development.

Implementation Details

The approach follows the same pattern used by the local explorer UI, with a reserved /cdn-cgi/ path intercepted by the entry worker and forwarded to the images service.

• CorePaths.IMAGE_DELIVERY and CoreBindings.SERVICE_IMAGES_DELIVERY have been added as entry worker constants.
• The entry worker routes /cdn-cgi/imagedelivery/ requests to the images service
• The images service fetch() handler serves raw bytes from KV using sharp.js for content type detection (via /info in the binding)
• The images service name is passed from the miniflare config assembler to the global services builder, which conditionally binds it to the entry worker
• Variant URLs are relative paths rather than absolute URLs as I'm not sure how we'd get fully qualified URLs to work with ports

Testing

Three new tests added for the delivery endpoint:

• Variant URLs use the correct /cdn-cgi/imagedelivery/ format
• Delivery endpoint serves uploaded image bytes
• Delivery endpoint returns 404 for non-existent images

Running my local test worker, the images now load:

Tests

• Tests included/updated
• Automated tests not possible - manual testing has been completed as follows:
• Additional testing not necessary because:
• Public documentation
  • Cloudflare docs PR(s):
  • Documentation not necessary because: this feature hasn't been released yet

---

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commit: 4e488eb

[workers-devprod]

Codeowners approval required for this PR:

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Show detailed file reviewers

• packages/miniflare/src/index.ts: [@cloudflare/wrangler]
• packages/miniflare/src/plugins/core/index.ts: [@cloudflare/wrangler]
• packages/miniflare/src/workers/core/constants.ts: [@cloudflare/wrangler]
• packages/miniflare/src/workers/core/entry.worker.ts: [@cloudflare/wrangler]
• packages/miniflare/src/workers/images/images.worker.ts: [@cloudflare/wrangler]
• packages/miniflare/test/plugins/images/index.spec.ts: [@cloudflare/wrangler]