[Fix] Telegram first messages lose attachments in new Sessions

[roomote-roomote]

​Opened on behalf of @daniel-lxs. Follow up by mentioning @roomote-roomote, in the web UI, or in Telegram.

What changed

• Preserve structured attachment text when Telegram schedules the first turn of a new Fast Session, including across durable retries.
• Treat Telegram photos and supported image documents as valid image input even when Telegram reports a generic MIME type, while extracting text documents, transcribing audio and voice notes, and describing supported video documents within existing size limits.
• Keep unsupported files undownloaded and document the supported Telegram attachment behavior.
• Add first-session regression coverage for every supported attachment category, unsupported documents, immediate admission, and durable event delivery.

Why this change was made

Telegram appended extracted non-image content to the visible question but dropped the structured current-turn attachment payload at the shared Fast surface-reply boundary. That prevented the first Session turn from retaining the same attachment context used by Fast's explicit task attachment handoff.

Impact

The first Telegram message in a Fast Session now retains supported images and bounded non-image context through both immediate execution and queued retries. Task attachment forwarding remains explicit and default-off, while unsupported files, webhook idempotency, topic routing, and existing task-entry behavior are unchanged.

[roomote-community]

No new code issues found. See task

• apps/api/src/handlers/telegram/attachments.ts:101 filename-recognized image documents with generic MIME types are emitted as non-image data URLs and dropped before Fast forwarding.

_{Reviewed 5b0aa72}

[roomote-roomote]

Resolved the outstanding image-document MIME fallback issue in 30a1c845. Generic Telegram/download MIME metadata now falls back to a valid image/png data URL, with focused regression coverage. See review task.