feat(db): add created_by column to chat_messages table (#22940)

Adds a `created_by` column (nullable UUID) to the `chat_messages` table
to track which user created each message. Only user-sent messages
populate this field; assistant, tool, system, and summary messages leave
it null.

The column is threaded through the full stack: SQL migration, query
updates, generated Go/TypeScript types, db2sdk conversion, chatd
(including subagent paths), and API handlers. All API handlers that
insert user messages now pass the authenticated user's ID as
`created_by`.

No foreign key constraint was added, matching the existing pattern used
by `chat_model_configs.created_by`.

coderd/database/queries/chats.sql

@@ -192,6 +192,7 @@ WITH updated_chat AS (
 )
 INSERT INTO chat_messages (
     chat_id,
+    created_by,
     model_config_id,
     role,
     content,
@@ -206,6 +207,7 @@ INSERT INTO chat_messages (
     compressed
 ) VALUES (
     @chat_id::uuid,
+    sqlc.narg('created_by')::uuid,
     sqlc.narg('model_config_id')::uuid,
     @role::text,
     sqlc.narg('content')::jsonb,