Add batch assign endpoint and functionality

[bcb37]

Adds the authorized /api/batch-assign endpoint, along with tests.
The new endpoint can be tested in swagger.

[danoswaltCL]

tested this out w/ swagger, I think the responses look right to me so far. I think I need to sit with the code for a bit, there's a lot going on i need to wrap my head around, or maybe the diff makes it seem like there's more than there really is.

One very nitty thing may be that there's a technical difference between a null when a user initialized but is not in the experiment, and null when the user is not initialized. You wouldn't know which is which in the response. it might not matter for the use-case this is being introduced for in rec-engine... but it could be a gotcha down the line. And this very much depends on stored users, which may complicate trying to do the ephemeral-user thing in experiments... just thinking out loud...

@msandbothe when you get a chance, can you take a look at this also? The responses will look like this, which is a bit different than what was specifically asked for because we wanted to keep the assignment object (the response is an Assignment type) the same as is used elsewhere, so the assignedCondition will be null if the user is not included. Otherwise, you'd want the payload here, so assignedCondition[0].payload.value. Maybe that's a little awkward, we usually are sending objects like this to a client library that parses it.

We wanted to keep this generic and consistent enough with other similar responses that it can be used for other general batch user queries.

{
  "user1": {
    "target": "faketarget3",
    "site": "fakesite3",
    "experimentType": "Simple",
    "assignedCondition": [
      {
        "id": "eb491cba-4da2-407b-89e6-d10dd9bf7f49",
        "conditionCode": "variant",
        "payload": {
          "type": "string",
          "value": "variant"
        },
        "experimentId": "544dd5b2-564d-4e12-9416-7aae99b2f354"
      }
    ]
  },
  "user2": null
}

[Copilot]

Pull Request Overview

This PR adds a new batch assignment endpoint /api/batch-assign that allows getting experiment conditions for multiple users in a single request. The implementation includes refactoring existing assignment logic to support batch processing, adding comprehensive validation, and providing extensive test coverage.

Key changes:

• Introduces getBatchExperimentConditions method for processing multiple users simultaneously
• Refactors assignment formatting logic into a reusable formatAssignments method
• Adds new repository methods for batch querying enrollments and exclusions

Reviewed Changes

Copilot reviewed 15 out of 15 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
backend/packages/Upgrade/src/api/controllers/BatchAssignController.ts	New controller implementing the batch assignment endpoint with validation and error handling
backend/packages/Upgrade/src/api/services/ExperimentAssignmentService.ts	Core batch assignment logic with segment resolution refactoring and formatting utilities
backend/packages/Upgrade/src/api/services/ExperimentUserService.ts	Added methods for retrieving multiple user documents
backend/packages/Upgrade/src/api/repositories/*.ts	Extended repositories with batch query methods for improved performance
backend/packages/Upgrade/test/unit/services/ExperimentAssignmentService.test.ts	Comprehensive test suite covering various batch assignment scenarios

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