feat: post-response chaos for proxy mode (phase 2 - non-streaming)

[marthakelly]

Summary

Adds post-response chaos for proxy mode (non-streaming). Enables mutating proxied responses after upstream call.

What’s included

• malformed now works in proxy mode
  • upstream is called
  • response body is corrupted before returning
• Works for non-streaming responses
• Preserves all existing behavior for fixtures and proxy

Implementation

• Intercepts res.write / res.end during proxy
• Buffers upstream response
• Applies chaos decision after proxy completes
• Replays or mutates buffered response
• Ensures response methods are always restored (fixes lifecycle bugs)

Why

Previously, proxy mode could not simulate response corruption — only request-level failures. This enables realistic downstream parsing failures when using real upstream APIs.

Scope

This PR covers Phase 2 (non-streaming):

• malformed ✅

Follow-up PR will include:

• status_override
• truncate
• streaming (SSE / NDJSON) support

Tests

• New: malformed chaos in proxy mode (invalid JSON)
• Full suite passing (~2400 tests)

Notes

• Minimal, contained implementation
• Does not modify proxy layer directly
• Uses response interception; can be refactored into cleaner abstraction later

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/@copilotkit/aimock@120

commit: bb46743

[marthakelly]

closing, trying another approach