fix(coderd): use pasted-text attachments as chat title input - #27067

Closes https://linear.app/codercom/issue/CODAGT-268

Problem

The chat UI collapses large pastes (>=10 lines or >=1000 chars) into a synthetic pasted-text-*.txt attachment. A chat created with only such an attachment had no title input anywhere: the create path derived titleSource only from text and file-reference parts (so the chat was named "New Chat"), async auto-titling extracted text the same way and silently skipped generation, and the manual propose/regenerate paths returned an empty title for the same reason. The regular prompt path already inlines these files for the model; only the title paths were blind.

Fix

Add a single title-input derivation in chatprompt and use it everywhere:

• chatprompt.TitleText joins text and file-reference parts (unchanged formatting), and falls back to synthetic pasted-text attachment content (truncated to a 16 KiB title budget) when they yield nothing.
• chatprompt.SyntheticPasteFileIDs identifies paste attachments; chatprompt.FallbackTitle consolidates the previously duplicated chatTitleFromMessage / fallbackChatTitle.
• Chat creation captures paste blob references while validating file parts (the file row was already loaded there) and derives titleSource via TitleText. Only the create path derives titles; message send and edit reuse the same validation without copying any blob data.
• GenerateChatTitleAsync and the manual propose/regenerate paths resolve paste content via titlePasteText, which only queries when a visible user message has no other title text, so chats with typed text never incur a file fetch.
• Title-path paste fetches are bounded: a new GetChatFileDataPrefixesByIDs query returns only a substr prefix (chatprompt.TitlePasteBytePrefix, 64 KiB = 4 bytes x the 16 Ki-rune title budget) so full blobs (up to 10 MiB each) never leave the database for titling, and chatprompt.TitlePasteText applies the same bound to the create path which already holds the loaded row.

Deliberate side effect: because generation-time extraction now matches create-time titleSource exactly, file-reference-only chats also become eligible for AI titles. They were previously skipped by the same derivation mismatch.

Non-goals: no frontend changes (attachment chip UX stays as is), and non-synthetic user-uploaded .txt files still yield "New Chat".

Testing

• Unit tests for TitleText, TitlePasteText, SyntheticPasteFileIDs, FallbackTitle, titleInput, titlePasteText, and paste-aware extractManualTitleTurns.
• Real-database test for GetChatFileDataPrefixesByIDs (prefix shorter and longer than stored data) plus dbauthz coverage for the new query.
• Integration tests: paste-only create gets a fallback title from the paste content, async title generation fires with the paste content as input, and RegenerateChatTitle works on a paste-only chat.

This PR was written by Mux on Mike's behalf.