[BREAKING] .NET: Decouple Checkpointing from Run/StreamAsync APIs

[lokitoth]

Motivation and Context

To support execution environments that do not support bring-your-own Checkpointing, we need to remove the ability to introduce CheckpointManager during a RunAsync / StreamAsync call.

Description

Change Checkpointing to be a configuration of IWorkflowExecutionEnvironment intrinsically. This changes how checkpointing is configured for InProcessExecutionEnvironment (now using WithCheckpointing)

BREAKING: Changes how Checkpointing is attached for InMemory execution.

Currently in draft form pending #3792

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[Copilot]

Pull request overview

This PR implements a breaking change to decouple checkpointing from the workflow execution APIs. Instead of passing a CheckpointManager as a parameter to RunAsync and StreamAsync methods, checkpointing is now configured as an intrinsic property of the IWorkflowExecutionEnvironment using the new WithCheckpointing() method on InProcessExecutionEnvironment.

Changes:

• Removes CheckpointManager parameters from all RunAsync, StreamAsync, and ResumeAsync methods in IWorkflowExecutionEnvironment
• Introduces WithCheckpointing() method on InProcessExecutionEnvironment to configure checkpointing
• Replaces Checkpointed<T> wrapper class with CheckpointableRunBase base class that both Run and StreamingRun now inherit from
• Updates WorkflowHostAgent and WorkflowSession to manage checkpointing configuration internally
• Adds IsCheckpointingEnabled property to the execution environment interface

Reviewed changes

Copilot reviewed 28 out of 28 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
IWorkflowExecutionEnvironment.cs	Removes checkpointing-related method overloads and adds IsCheckpointingEnabled property
InProcessExecutionEnvironment.cs	Adds WithCheckpointing() method and internal checkpoint manager configuration
InProcessExecution.cs	Updates static helper methods to use WithCheckpointing() pattern
Checkpointed.cs	Converts from generic wrapper to base class CheckpointableRunBase
Run.cs / StreamingRun.cs	Changes to inherit from CheckpointableRunBase instead of being wrapped by Checkpointed<T>
WorkflowSession.cs	Updates to configure checkpointing on the execution environment rather than passing it to run methods
WorkflowHostAgent.cs	Removes checkpointing parameter from constructor and validation logic
InProcessRunner.cs	Adds checkpoint index restoration and IsCheckpointingEnabled property
CheckpointManager.cs / ICheckpointManager.cs	Adds RetrieveIndexAsync method for retrieving checkpoint collections
Test files	Updates all test code to use new checkpointing pattern
Sample files	Updates all sample code to use new checkpointing pattern