Keep inaccessible file refs visible in web chat

[zzj3720]

Summary

• render fileRef-only user attachments instead of dropping the card
• show an incomplete-content fallback card for web-inaccessible file refs with file location and environment, including image/audio/video/file refs
• add an action button that asks the agent to provide a complete/downloadable version and disables after request is sent
• share the fallback predicate across user/assistant renderers and cover fallback/request behavior with tests

Validation

• yarn typecheck
• yarn lint
• yarn test
• yarn build:embedded (passes with existing bundle-size warnings)
• Manual: rendered the incomplete file card path-only local-vm scenario in jsdom and clicked the “Ask agent to make accessible” button to confirm the user-visible state, generated agent request text, and disabled sent state