Remove per-request JSON deep copy from getCachedValidExperiments

[danoswaltCL]

I'm leaving this one in draft for now because i need to test it more, but copilot can give it a crack to validate claude's claims:

per Claude:

getCachedValidExperiments wrapped every cache hit in JSON.parse(JSON.stringify(...)). That defensive copy existed because formattingConditionPayload mutated the experiment it was given — it deleted conditionPayloads off the cached conditions/partitions in place. Since the cache is in-memory and hands back the same object reference to every request, mutating there would corrupt the cached graph for all subsequent requests.

formattingConditionPayload now rebuilds the stripped conditions and partitions as new objects instead of deleting fields in place, so the cached graph is never written to and the per-request deep copy is gone. The parentCondition/decisionPoint back-references point at the rebuilt objects, so reference identity within the returned experiment matches what the in-place version produced.

Five tests in ExperimentService.test.ts guard the non-mutation invariant that makes removing the copy safe, covering both the simple and factorial paths.

[Copilot]

Pull request overview

This PR removes the per-request JSON.parse(JSON.stringify(...)) deep copy from ExperimentService.getCachedValidExperiments() by updating formattingConditionPayload() to avoid mutating cached experiment objects, and adds unit tests to guard the non-mutation invariant.

Changes:

• Removed the defensive JSON deep copy on cache hits in getCachedValidExperiments().
• Reworked formattingConditionPayload() to rebuild stripped conditions/decision points as new objects rather than deleting fields in place.
• Added unit tests verifying formattingConditionPayload() does not mutate its input and maintains expected reference/back-reference behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
packages/backend/src/api/services/ExperimentService.ts	Removes cache-hit deep copy and updates formattingConditionPayload() to avoid mutating cached graphs.
packages/backend/src/api/services/ExperimentAssignmentService.ts	Updates inline documentation about cached experiment mutation expectations.
packages/backend/test/unit/services/ExperimentService.test.ts	Adds non-mutation and stability tests for formattingConditionPayload().

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.