Placeholder is assigned to an app before worker is initialized

Created for [this incident](https://portal.microsofticm.com/imp/v3/incidents/details/484018781/home) (internal-only link). This appears to be a rare edge case. For this particular app, it only happened once in the last 90 days.

We see a Node.js worker starting on a placeholder template site, but rather than getting the WorkerInitRequest while still assigned to the placeholder app, the worker gets both a WorkerInitRequest and a FunctionEnvironmentReloadRequest while assigned to the actual customer app. The Node.js worker didn't handle this well. I filed [an issue](https://github.com/Azure/azure-functions-nodejs-worker/issues/737) on the Node.js side of things as well, but I don't think the host (or whoever) should be using a placeholder site for a user's site until after worker init has finished.

Here's a very pared down version of the logs, see the incident for an actual query:

| PreciseTimeStamp | AppName | Summary |
| --- | --- | --- |
|2024-02-14T10:17:15.2232391Z | functionsv4x64placeholdertemplatesite | Initiating Worker Process start up |
|2024-02-14T10:17:15.2251832Z | functionsv4x64placeholdertemplatesite | Starting worker process with FileName:node ... |
|2024-02-14T10:17:15.310154Z | <customer app> | node process with Id=4976 started |
|2024-02-14T10:17:16.7860431Z | <customer app> | Worker Process started. Received StartStream message |
|2024-02-14T10:17:16.8283348Z | <customer app> | Worker be917f1c-3ae1-4142-ac5d-29e7597e1678 received WorkerInitRequest |
|2024-02-14T10:17:17.2637967Z | <customer app> | Received WorkerInitResponse. Worker process initialized |
|2024-02-14T10:17:17.2721214Z | <customer app> | Sending FunctionEnvironmentReloadRequest to WorkerProcess with Pid: '4976' |
|2024-02-14T10:17:17.3279711Z | <customer app> | Received FunctionEnvironmentReloadResponse from WorkerProcess with Pid: '4976' |

Start by reviewing the incident details and the linked Node.js worker issue, since no source files, tests, or entry points are named here. Trace how placeholder app assignment overlaps WorkerInitRequest and FunctionEnvironmentReloadRequest, then verify that a customer app is not assigned before worker initialization completes.

Written by the indexing model from the issue text.