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](https://mux.coder.com) on Mike's behalf.

coderd/database/dbmock/dbmock.go

@@ -2878,6 +2878,21 @@ func (mr *MockStoreMockRecorder) GetChatFileByID(ctx, id any) *gomock.Call {
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetChatFileByID", reflect.TypeOf((*MockStore)(nil).GetChatFileByID), ctx, id)
 }
 
+// GetChatFileDataPrefixesByIDs mocks base method.
+func (m *MockStore) GetChatFileDataPrefixesByIDs(ctx context.Context, arg database.GetChatFileDataPrefixesByIDsParams) ([]database.GetChatFileDataPrefixesByIDsRow, error) {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "GetChatFileDataPrefixesByIDs", ctx, arg)
+	ret0, _ := ret[0].([]database.GetChatFileDataPrefixesByIDsRow)
+	ret1, _ := ret[1].(error)
+	return ret0, ret1
+}
+
+// GetChatFileDataPrefixesByIDs indicates an expected call of GetChatFileDataPrefixesByIDs.
+func (mr *MockStoreMockRecorder) GetChatFileDataPrefixesByIDs(ctx, arg any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "GetChatFileDataPrefixesByIDs", reflect.TypeOf((*MockStore)(nil).GetChatFileDataPrefixesByIDs), ctx, arg)
+}
+
 // GetChatFileMetadataByChatID mocks base method.
 func (m *MockStore) GetChatFileMetadataByChatID(ctx context.Context, chatID uuid.UUID) ([]database.GetChatFileMetadataByChatIDRow, error) {
 	m.ctrl.T.Helper()