Update config

[LuceCarter]

This pull request makes several improvements to configuration, startup behavior, and documentation for the MongoDB .NET example project. The main focus is to move configuration from environment variables to strongly-typed settings in appsettings.json, modernize the health check endpoint, and clarify documentation and devcontainer setup. It also introduces a new settings model for startup behavior and updates related code and docs accordingly.

Configuration and Startup Behavior Modernization

• Introduced a new StartupBehaviorSettings model, moving the SeedOnStartup flag from the SEED_ON_STARTUP environment variable to a strongly-typed StartupBehaviorSettings:SeedOnStartup entry in appsettings.json and appsettings.Development.json. Updated Startup.cs to use this setting via dependency injection, removing direct environment variable reads.

• Updated documentation (README.md, AGENTS.md, EDD.md) to reflect the new configuration approach, replacing references to environment variables with app settings sections and keys. Clarified that environment variables are now optional overrides.

Health Check Endpoint Update

• Changed the health check endpoint from /healthz to /health in both code and documentation for consistency and convention.

Devcontainer and Environment Improvements

• Updated .devcontainer/devcontainer.json to set port 5000 visibility to public, improving GitHub Codespaces usability.
• Changed the devcontainer Docker image tag for better alignment with current .NET devcontainer images.

Solution and Project Structure

• Added test project references to the solution file, organizing tests under a dedicated solution folder.

These updates collectively make configuration more robust and discoverable, improve developer onboarding, and align the project with modern .NET best practices.