Graceful Shutdown

Currently, when an **ASP.NET Core** application running **Elsa Workflows** shuts down, it may terminate workflows that are still executing. This can lead to inconsistencies, especially when workflows interact with external systems, databases, or queues.

This feature request proposes a **graceful shutdown mechanism** that allows active workflows to **complete up to a commit point** before the application stops while ensuring that **no new workflows are started during the shutdown phase**.

### **Use Case & Requirements**
1. **Ongoing Workflows Complete Before Shutdown**
- Elsa should detect when the application is shutting down.
- Workflows that are currently executing should be allowed to **finish their current activity but not proceed beyond a commit point**.

2. **New Workloads Should Not Be Accepted**
- MassTransit consumers should **stop accepting new messages** while allowing in-flight messages to complete.
- Background jobs (e.g., **Quartz.NET, Hangfire**) should **pause scheduling new work**.
- HTTP API endpoints for starting new workflows should **return a service unavailable response** or **queue execution until restart**.

3. **Extensibility for 3rd-Party Modules**
- Introduce a **global shutdown notification event** that extensions can hook into.
- Provide an interface like **`IShutdownHandler`** that modules can implement to clean up background tasks and other resources.

### **Proposed Implementation (by chatGPT)**
- Introduce a **shutdown token** or **state flag** that Elsa’s execution engine checks before starting new activities.
- Implement a **shutdown event dispatcher** that:
- Stops MassTransit consumers.
- Pauses job schedulers.
- Notifies registered `IShutdownHandler` implementations.
- Ensure the `WorkflowRunner` (or equivalent execution engine) respects shutdown signals.

### **Impact**
This would:
- Improve reliability when shutting down applications running Elsa Workflows.
- Ensure workflows reach a **natural stopping point** before termination.
- Provide a **consistent and extensible** mechanism for module developers.

Start by mapping the WorkflowRunner or equivalent execution engine against the application shutdown signal, then review how MassTransit consumers, Quartz.NET, Hangfire, and HTTP workflow-start endpoints accept work. Consider the proposed shutdown token, global notification event, and IShutdownHandler entry points. Done means active workflows reach a commit point, new work is rejected or paused, and extensions can clean up consistently.

Written by the indexing model from the issue text.