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/test/mysql-migration.spec.ts

@@ -1,9 +1,10 @@
 import path from "path";
 import { MySqlContainer } from "@testcontainers/mysql";
+import { eq } from "drizzle-orm";
 import { drizzle } from "drizzle-orm/mysql2";
 import { migrate } from "drizzle-orm/mysql2/migrator";
 import mysql from "mysql2";
-import { describe, test } from "vitest";
+import { describe, expect, test } from "vitest";
 
 import { DB_CASING } from "@homarr/core/infrastructure/db/constants";
 
@@ -35,6 +36,22 @@ describe("Mysql Migration", () => {
     // Check if users table exists
     await database.query.users.findMany();
 
+    // A single base64 image attachment already exceeds the 64KB that `text` holds on MySQL, so
+    // `assistant_message.content` has to be `mediumtext`. Everything below 64KB passes either way.
+    const largeContent = "x".repeat(200_000);
+    await database.insert(mysqlSchema.users).values({ id: "size-check-user", name: "size-check" });
+    await database.insert(mysqlSchema.assistantThreads).values({ id: "size-check-thread", userId: "size-check-user" });
+    await database.insert(mysqlSchema.assistantMessages).values({
+      id: "size-check-message",
+      threadId: "size-check-thread",
+      parentId: null,
+      content: largeContent,
+    });
+    const [storedMessage] = await database.query.assistantMessages.findMany({
+      where: eq(mysqlSchema.assistantMessages.id, "size-check-message"),
+    });
+    expect(storedMessage?.content).toHaveLength(largeContent.length);
+
     connection.end();
     await mysqlContainer.stop();
   }, 120_000);