fix(db): store assistant messages as mediumtext on mysql

MySQL caps `text` at 65,535 bytes. A single base64 image attachment is
allowed up to ~1.34MB by the chat route, so appending such a message fails
with ER_DATA_TOO_LONG under the default strict mode. SQLite and PostgreSQL
are unaffected because their text types are effectively unbounded.

`mediumtext` holds 16MB, comfortably above the 2MB request cap.

The migration keeps the parenthesised default expression that every other
text default in this folder uses, because MySQL rejects a literal default
on TEXT columns.

packages/db/schema/mysql.ts

@@ -8,6 +8,7 @@ import {
   customType,
   index,
   int,
+  mediumtext,
   mysqlTable,
   primaryKey,
   smallint,
@@ -536,7 +537,9 @@ export const assistantMessages = mysqlTable(
       .references(() => assistantThreads.id, { onDelete: "cascade" }),
     parentId: varchar({ length: 128 }),
     format: varchar({ length: 64 }).notNull().default("ai-sdk/v6"),
-    content: text().default(emptySuperJSON).notNull(),
+    // `text` only holds 64KB on MySQL, which a single base64 image attachment already exceeds.
+    // `mediumtext` holds 16MB, comfortably above the 2MB request cap enforced by the chat route.
+    content: mediumtext().default(emptySuperJSON).notNull(),
     createdAt: timestamp().notNull().defaultNow(),
   },
   (message) => ({