fix(agent): retry oversized resume on a fresh session with summarized history

[tatoalo]

Problem

When a resumed session's transcript no longer fits the model's context window, the API rejects every request — including the SDK's own auto-compaction — so the resume continuation fails with Internal error: Prompt is too long. That is classified as a non-recoverable agent_error, the run dies before doing any work, and every subsequent resume of the task hits the same wall: the task is permanently stuck.

Follow-up to #3331 (which makes hydration unlikely to produce an oversized transcript in the first place); this adds the safety net for any remaining path to an oversized resume.

Changes

• When the resume continuation fails with a prompt-too-long error, start a fresh session (same session meta) and re-send the pending user message with the summarized conversation history that the non-native resume path already uses (capped at 50k estimated tokens), instead of failing the run.
• One retry per run, gated to the native-resume continuation turn; any other failure keeps the existing handleTurnFailure behavior.
• New isPromptTooLongError predicate in error-classification.ts.

How did you test this?

• New test: a resume continuation whose first prompt rejects with "Prompt is too long" creates a fresh session, swaps the active ACP session id, and re-prompts with the summarized history.
• pnpm exec vitest run src/server/agent-server.test.ts — 101 passed.
• tsc --noEmit on the agent package.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog Code

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit dbe76da.}