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/querier_test.go

@@ -1,6 +1,7 @@
 package database_test
 
 import (
+	"bytes"
 	"context"
 	"database/sql"
 	"encoding/json"
@@ -1918,6 +1919,56 @@ func TestGetAuthorizedChatsByChatFileIDACLSharing(t *testing.T) {
 	require.Empty(t, rows[0].GroupACL)
 }
 
+func TestGetChatFileDataPrefixesByIDs(t *testing.T) {
+	t.Parallel()
+	if testing.Short() {
+		t.SkipNow()
+	}
+
+	ctx := testutil.Context(t, testutil.WaitMedium)
+	sqlDB := testSQLDB(t)
+	err := migrations.Up(sqlDB)
+	require.NoError(t, err)
+	db := database.New(sqlDB)
+
+	owner := dbgen.User(t, db, database.User{})
+	org := dbgen.Organization(t, db, database.Organization{})
+
+	longData := bytes.Repeat([]byte("a"), 100)
+	longFile, err := db.InsertChatFile(ctx, database.InsertChatFileParams{
+		OwnerID:        owner.ID,
+		OrganizationID: org.ID,
+		Name:           "long.txt",
+		Mimetype:       "text/plain",
+		Data:           longData,
+	})
+	require.NoError(t, err)
+	shortFile, err := db.InsertChatFile(ctx, database.InsertChatFileParams{
+		OwnerID:        owner.ID,
+		OrganizationID: org.ID,
+		Name:           "short.txt",
+		Mimetype:       "text/plain",
+		Data:           []byte("tiny"),
+	})
+	require.NoError(t, err)
+
+	rows, err := db.GetChatFileDataPrefixesByIDs(ctx, database.GetChatFileDataPrefixesByIDsParams{
+		IDs:         []uuid.UUID{longFile.ID, shortFile.ID},
+		PrefixBytes: 16,
+	})
+	require.NoError(t, err)
+	require.Len(t, rows, 2)
+
+	prefixes := make(map[uuid.UUID]database.GetChatFileDataPrefixesByIDsRow, len(rows))
+	for _, row := range rows {
+		prefixes[row.ID] = row
+	}
+	require.Equal(t, longData[:16], prefixes[longFile.ID].DataPrefix)
+	require.Equal(t, []byte("tiny"), prefixes[shortFile.ID].DataPrefix)
+	require.Equal(t, owner.ID, prefixes[longFile.ID].OwnerID)
+	require.Equal(t, org.ID, prefixes[longFile.ID].OrganizationID)
+}
+
 func TestInsertWorkspaceAgentLogs(t *testing.T) {
 	t.Parallel()
 	if testing.Short() {