[Fix] Unsupported prompt attachments disappear without feedback

[roomote-roomote]

​Opened on behalf of @daniel-lxs. View the task or mention @roomote-roomote for follow-up asks.

Related issue

No linked issue; maintainer-authorized bounded product investigation.

Why this PR exists

• A maintainer explicitly invited this PR in the linked issue or discussion
• I am a maintainer / this is internal Roomote work

Unsupported attachments disappeared silently from the web composer. Selecting a mixed batch also silently omitted unsupported files, leaving users unsure which context would be included.

What changed

Show the existing attachment-validation error as a toast. Mixed batches now report that some file types were not added while preserving supported attachments. Prompt text, accepted types, extraction, submission and transport behavior are unchanged.

How it was tested

Base d95a5fd045ff82f81d458256a6a871ec9cda611a; exact head d5415bb8928d5f4d68888ade0026c18a1ba80ff3.

Real browser baseline on the authenticated home composer: choose a synthetic .bin file and observe no chip or notification; choose .txt and observe its chip. A real mixed .txt/.bin selection retained only the text file without explaining the omission. The initial fix showed the rejection toast on desktop and mobile, preserved the prompt, and allowed a supported file to be added afterward.

Draft awaiting proof. The shared visual-proof window timed out before final-head mixed-batch proof. Earlier captures predate the mixed-batch change and are intentionally not presented as final screenshots. Capture and recovery stopped when the deadline was reached.

Resumable validation on this exact head: sign in to the local app, enter prompt text, select an unsupported .bin, confirm the rejection toast and preserved text, then select .txt plus .bin together and confirm one supported chip plus the partial-rejection toast. Remove/re-add the supported file and repeat at desktop and mobile widths. Capture the final-head states before marking the proof complete. No inference or task launch is needed for this admission-only claim.

• The rejection-notice regression failed before the fix. Final focused run passed 37 tests across TaskPromptInput, shared PromptInput and Home: pnpm exec dotenvx run -f .env.test -- pnpm --filter @roomote/web exec vitest run src/components/tasks/TaskPromptInput.client.test.tsx src/components/ai-elements/prompt-input.test.tsx 'src/app/(authenticated)/home/Home.client.test.tsx'.
• Web fast typecheck, changed-file formatting and diff checks passed. Actual pre-push hooks passed full oxlint, residual ESLint, fast typechecks and knip (non-failing existing mint hint).
• Completion review found no code gaps and confirmed the earlier images show initial toast/recovery behavior, but final visual proof is explicitly incomplete.
• PDF extraction, provider inference, durable Session submission, full repository tests and deployed behavior were not validated or changed. A separate local PDF probe hit a missing PDF worker dependency; no PDF parsing defect is claimed here.

Related PRs

• [Fix] Doctor reports blank auth credentials as configured #2300: independent doctor diagnostic correction from the preceding bounded wave. No code dependency; this branch starts directly from develop.

Checklist

• The PR title follows the repo convention: [Fix], [Feat], [Improve], [Refactor], [Docs], or [Chore] followed by a user-facing description
• This PR is small and scoped to one change
• pnpm lint and pnpm check-types pass locally
• I added tests or included a clear manual validation note above
• I removed secrets, tokens, private keys, and customer data from code, logs, and screenshots
• If this change should appear in the changelog, I ran pnpm changeset

Full pre-push static gates passed; the exact formatting-inclusive full lint and full tsc commands were not both run. No changeset added in this bounded fix.

[roomote-community]

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_{Reviewing d5415bb}