.NET: Migrate remaining Foundry hosted samples to source deployment

[rogerbarreto]

The first source deployment migration in #7372 established code and ZIP upload as the default path for Hosted-ChatClientAgent. The remaining .NET Foundry hosted agent samples still use Dockerfiles, container registry configuration, and the older split agent manifests.

Migrate the remaining samples to the same documented azd flow:

1. Use one azure.yaml with codeConfiguration and remote build.
2. Make every uploaded project self-contained because repository build files do not travel in the ZIP.
3. Remove obsolete Dockerfiles and split agent manifests from source deployment samples.
4. Preserve each sample's required environment variables, bundled resources, permissions, and external resource prerequisites.
5. Document local testing, deployment, invocation, contributor package flow, and cleanup per sample.
6. Validate each sample locally and against a Foundry project where its external dependencies are available.