test(frontend/copilot): add SSE streaming integration test infrastructure (#13098)

### Why / What / How

**Why:** The CoPilot frontend streams responses through AI SDK v6 via
SSE, but we had no integration tests exercising the streaming pipeline
end-to-end. The existing `CopilotPage.test.tsx` stubs out
`ChatContainer` entirely, so the wiring between user-message → transport
→ AI-SDK runtime → DOM-rendered assistant text was untested.

**What:** Adds two pieces of test infrastructure plus a happy-path
integration test:

1. `src/tests/integrations/copilot-sse.ts` — a reusable MSW-compatible
SSE response builder that emits AI-SDK v6 `UIMessageChunk` frames
(`text-start` / `text-delta` / `text-end` / `finish-step` / `finish`)
wrapped in the `text/event-stream` + `x-vercel-ai-ui-message-stream: v1`
wire format, terminated with `data: [DONE]`.
2. `copilotStreamHandler({ baseUrl, sessionId, chunks })` — convenience
factory that returns an MSW `http.post` handler matching the CoPilot
transport's absolute backend URL (the transport bypasses the Next.js
proxy to dodge Vercel's 800s function-timeout).
3. `copilotStream.integration.test.tsx` — happy-path test: mounts a tiny
harness component that wires `useCopilotStream` exactly like
`CopilotChatHost` does, clicks send, and asserts the streamed assistant
text renders.

**How:** The SSE helper builds a `ReadableStream<Uint8Array>` by
JSON-encoding each `UIMessageChunk` as a `data: …\n\n` frame. MSW v2
accepts `ReadableStream` bodies on `HttpResponse`, so no node-streams
adapter is needed. The test only mocks `getCopilotAuthHeaders` (to skip
Supabase) and overrides `environment.getAGPTServerBaseUrl()` to a fixed
host so the absolute URL is deterministic — everything else (transport,
AI SDK `Chat` runtime, registry, dedup, watchdogs) is real.

### Changes 🏗️

- New: `autogpt_platform/frontend/src/tests/integrations/copilot-sse.ts`
— `streamSseResponse`, `assistantTextChunks`, `copilotStreamHandler`.
- New:
`autogpt_platform/frontend/src/app/(platform)/copilot/__tests__/copilotStream.integration.test.tsx`
— two tests covering single-delta and multi-delta concatenation.

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
- [x] `pnpm exec vitest run
src/app/(platform)/copilot/__tests__/copilotStream.integration.test.tsx`
— 2 tests pass in <1s.
- [x] `pnpm exec vitest run src/app/(platform)/copilot` — full CoPilot
suite (884 tests across 56 files) still green.
  - [x] `pnpm exec tsc --noEmit` — clean.
  - [x] `pnpm prettier --write` applied to new files.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>