Components.AI: Support scalable attachment transfer, progress, and cancellation

### Is your feature request related to a problem? Please describe the problem.

Files are fully buffered into `byte[]` and emitted as base64 data URIs, which can cross Blazor Server circuits multiple times and cannot report progress or cancel per attachment.

### Describe the solution you'd like

Provide a streaming upload or application-owned URI/reference path, expose upload progress, and support per-attachment cancellation without requiring all content to be buffered in component memory.

### Additional context

Leaf task for #68227 requirements D6-D7.

Start by reading parent issue #68227, especially requirements D6-D7, then trace the Components.AI attachment-transfer implementation responsible for buffering files and emitting base64 data URIs. Define the supported streaming or application-owned reference path, progress reporting, and per-attachment cancellation behavior; done means attachments no longer require full component-memory buffering and all three capabilities are specified and covered.

Written by the indexing model from the issue text.