refactor: identify and freeze messages at creation

examples/headless-agent/tests/compaction.e2e.ts

@@ -1,3 +1,4 @@
+import { createUserMessage } from '@deepseek-ai/dsh-llm'
 import { mkdtemp, rm, writeFile } from 'node:fs/promises'
 import { tmpdir } from 'node:os'
 import { join } from 'node:path'
@@ -46,12 +47,13 @@ describe.skipIf(!process.env.DEEPSEEK_API_KEY)('compaction: a long session compa
     })
     const agent = ctx.agentLoop.create(SessionId('e2e-compaction'), { provider: 'deepseek', model: 'deepseek-v4-flash' })
 
-    agent.followup({ content: [{
-      type: 'text',
-      text: 'Read file1.txt, file2.txt, file3.txt, and file4.txt one at a '
+    agent.followup(createUserMessage({
+      content: [{
+        type: 'text',
+        text: 'Read file1.txt, file2.txt, file3.txt, and file4.txt one at a '
         + 'time using cat (a separate bash command for each). After reading all four, tell me how '
         + 'many files you read and the number mentioned in file1.txt.',
-    }], source: { kind: 'user' } })
+      }], source: { kind: 'user' } }))
     await waitForIdle(ctx, agent)
 
     const events = [...agent.session.events]