chore(internal): fix devcontainers setup

[stainless-app]

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EntelligenceAI PR Summary

The development environment setup has been simplified by removing the .devcontainer/Dockerfile and updating devcontainer.json to utilize a pre-built image and add VSCode extensions.

[entelligence-ai-pr-reviews]

Walkthrough

The .devcontainer/Dockerfile was removed, and the devcontainer.json was updated to use a pre-built image and include VSCode extensions. These changes streamline the development environment setup.

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