[codespaces] vscode plugin is installed by default but still get prompt to install it

[danieljurek]

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This probably needs to be routed to someone on vscode team.

The azd vscode extension is in the devcontainer.json file (example) yet we sometimes see alerts to install it.

Repro steps:

1. Create new codespaces instance of a repo (e.g. https://github.com/Azure-Samples/todo-nodejs-mongo/tree/main)
2. Wait for creation to finish
3. Observe first run

Expected: no alert to install azure developer extension Actual: Alert to install azure developer extension, after the extension is installed