.NET: Fix InMemoryCheckpointManager serialization

[lokitoth]

Motivation and Context

Checkpointing is used by the WorkflowHostAgent to be able to support resume from a provided thread. When a CheckpointManager is not specified, we use the InMemoryCheckpointManager and serialize its state into the thread's Serialize()ed JsonElement.

At some point InMemoryCheckpointManager became not serializable, breaking this behaviour. This change restores serializability and adds a test.

Closes #1629

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 fixes a serialization issue with InMemoryCheckpointManager that was preventing checkpoint state from being properly persisted and restored. The changes refactor internal fields to use public properties with [JsonInclude] attributes, add a new CheckpointInfoConverter for dictionary key serialization, and relocate shared escape/unescape utility methods to a base class.

Key changes:

• Made InMemoryCheckpointManager and RunCheckpointCache serializable by converting private fields to internal properties with JSON attributes
• Created CheckpointInfoConverter to handle CheckpointInfo as dictionary keys during serialization
• Moved escape/unescape helper methods from ScopeKeyConverter to the base class JsonConverterDictionarySupportBase

Reviewed Changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
JsonSerializationTests.cs	Added test to verify InMemoryCheckpointManager serialization round-trip and refactored existing checkpoint validation logic
InMemoryCheckpointManager.cs	Changed private _store field to public Store property with [JsonInclude] for JSON serialization
RunCheckpointCache.cs	Changed private _checkpointIndex and _cache fields to public properties with [JsonInclude]
CheckpointInfoConverter.cs	New converter enabling CheckpointInfo to be used as dictionary keys in JSON
JsonMarshaller.cs	Registered CheckpointInfoConverter in the internal serialization options
JsonConverterDictionarySupportBase.cs	Added shared Escape and Unescape methods with configurable escape character
ScopeKeyConverter.cs	Removed Escape and Unescape methods (moved to base class) and unused import
ExecutorIdentityConverter.cs	Added missing imports for updated converter implementation