Inserter media: save item metadata on the uploaded attachment

[thrijith]

What?

When an image from an inserter media category (Openverse or one registered via registerInserterMediaCategory()) is uploaded to the media library, the item's title, alt, and new optional description are now saved on the attachment. A new optional filename property sets the uploaded file's name. data:/blob: URLs no longer have a filename derived from them.

Fixes #60378.

Why?

Only caption was forwarded to the upload. Title and alt text reached the block but not the attachment, so the media library filled with items that had no alt text and a URL-derived title and filename (e.g. image-n.png for generated images). Image-generation providers like ClassifAI (see 10up/classifai#729) know the prompt at insert time and need a way to store it as the attachment's title, alt text, and description.

How?

• MediaPreview's onMediaInsert adds alt_text, title, and description to the upload's additionalData, each only when the item provides a value, so server-side defaults still apply otherwise.
• New optional filename and description properties on InserterMediaItem.
• The uploaded file is named media.filename || getFilename( url ) || 'image.jpg', skipping getFilename() for data:/blob: URLs since they contain no filename. Previously the derived name for a data: URL had no valid extension and the upload could fail.

Known limitation: drag-and-drop from the media tab uses the image block's sideload path and doesn't carry item metadata; that needs cross-package plumbing and is left as a follow-up.

Testing Instructions

1. Register a test category in the browser console on a post editor screen:

   wp.data.dispatch( 'core/block-editor' ).registerInserterMediaCategory( {
       name: 'test-media', labels: { name: 'Test', search_items: 'Search' },
       mediaType: 'image', isExternalResource: true,
       async fetch() { return [ {
           sourceId: 1, title: 'My Title', alt: 'My alt text',
           description: 'My description', filename: 'my-image.jpg',
           url: '<same-origin image URL>', previewUrl: '<same-origin image URL>',
       } ]; },
   } );

2. Inserter → Media → Test → click the item; wait for "Image uploaded and inserted."

3. Media Library → newest attachment: file is my-image.jpg; title, alt text, and description match the item.

4. Repeat with a data:image/png;base64,… url and no filename: upload succeeds, file named image.png.

5. Openverse: insert a CORS-allowed result (e.g. "Vincent van Gogh sunflowers painting") — attachment title matches the item title.

6. npm run test:unit -- packages/block-editor/src/components/inserter/media-tab/test/media-preview.js — 6 tests pass.

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