Research: Add VCR-style record/replay mode

[haasonsaas]

Context

VCR-pattern libraries (VCR.py, polly.js, go-vcr) provide "record real API calls, replay them deterministically" patterns for integration testing.

What to yoink

• Record mode — proxy real API calls to OpenAI/Anthropic, capture request/response pairs as "cassettes"
• Replay mode — match incoming requests against recorded cassettes, return deterministic responses
• Request matching — configurable matching (exact, fuzzy, header-ignoring) for finding the right recorded response
• Cassette management — versioned, human-readable recording files (YAML/JSON) that can be committed to repos

Approach

Mocktopus is already a drop-in mock for OpenAI/Anthropic APIs. Adding VCR-style recording makes it useful for a different workflow:

1. Record real API interactions during manual testing
2. Replay them in CI for deterministic, zero-cost integration tests
3. Re-record when prompts or model behavior changes

This complements the existing mock approach — mocks for unit tests, VCR for integration tests.

References

• https://github.com/kevin1024/vcrpy
• https://github.com/Netflix/pollyjs
• https://github.com/dnaeon/go-vcr

Priority

Tier 3 — Complementary testing mode