.NET: add container-hosted workflow sample

[Naveen (1aifanatic)]

Motivation & Context

Developers need a runnable example of a pro-code workflow hosted outside Foundry. This sample exposes a small expense-routing workflow over HTTP and builds it into a non-root Linux container. Deterministic executors make deployment verification possible without model credentials.

Description & Review Guide

• What are the major changes? Add an ASP.NET Core sample with two executors, request validation, per-request workflow instances, and run disposal. Include a multi-stage Dockerfile, a Dockerfile-specific context filter, local/container instructions, and solution/sample-index entries.
• What is the impact of these changes? No framework APIs or runtime behavior change. A local net10.0 build completed with zero warnings/errors. The Docker image built and ran as UID 1654. Both local and container hosts passed health, review-threshold, invalid-input, and 32-concurrent-request isolation checks. The container build emitted SourceLink warnings because Git metadata is intentionally excluded. Repository-wide unit tests were not run for this sample-only change.
• What do you want reviewers to focus on? Whether this is the preferred home and scope for the outside-Foundry pro-code deployment example. The README identifies in-process persistence, authentication, and repeated-request boundaries; the sample does not execute payments or claim durable recovery.

Related Issue

Contributes to #2056. This is one of the four requested deployment scenarios; it intentionally does not close the umbrella issue. No competing open PR for this sample was found during the preflight.

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

🟡 Changes recommended

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Pull request overview

Adds a runnable ASP.NET Core sample demonstrating container-hosted, deterministic .NET workflows.

Changes:

• Adds expense-routing workflow endpoints with validation and isolated runs.
• Adds a non-root multi-stage container build.
• Documents usage and registers the sample.

File summaries

File	Description
dotnet/samples/04-hosting/README.md	Lists the new sample.
dotnet/samples/04-hosting/ContainerWorkflow/README.md	Documents usage and hosting boundaries.
dotnet/samples/04-hosting/ContainerWorkflow/Program.cs	Implements the HTTP workflow host.
dotnet/samples/04-hosting/ContainerWorkflow/Dockerfile.dockerignore	Filters container build context.
dotnet/samples/04-hosting/ContainerWorkflow/Dockerfile	Builds the non-root runtime image.
dotnet/samples/04-hosting/ContainerWorkflow/ContainerWorkflow.csproj	Defines the sample project.
dotnet/agent-framework-dotnet.slnx	Registers the project in the solution.

Review details

• Files reviewed: 7/7 changed files
• Comments generated: 0
• Review effort level: Balanced

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