.NET: fix: Make State Persistence APIs work better with PortableValue

[lokitoth]

Motivation and Context

With the changes to make our type system work a better with transparent serialization round-tripping during checkpointing, we replaced the object-based catch-all in MessageRouter and RouteBuilder with one taking PortableValue, since object as a target-type can result in truncation or leaking JsonElement objects when interacting with JSON serialization.

We did not make a similar change to state persistence, which means that it is unclear how to get a "supertype" of everything to operate on when a specific type is not known. Users could get into trouble writing arbitrary types and trying to read back object. They could use PortableValue but this caues

It also exposed a pair of bugs:

• in cases like the abstraction leak detailed above, the deserialization process caches the bad leaked value and makes it impossible to cleanly deserialize as the correct value afterwards.
• in cases where the user asks for the wrong type backed by JSON via IDelayedDeserialization (and would leak if requested to convert to object), PortableValue throws out of Is<T>/As<T>.

Description

• QOL improvement for developer: Now loading a state as PortableValue will correctly wrap non-PV objects in it, but also avoid double-wrapping if it was stored as a PV in the first place.
• Fix the two bugs in PortableValue

Closes #1361

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 improves state persistence APIs to work better with PortableValue by fixing serialization round-trip issues and preventing object type abstractions that can leak JsonElement objects. The changes ensure PortableValue can serve as a supertype for all stored values without double-wrapping or deserialization cache corruption.

Key changes:

• Enhanced StateManager and StateScope to properly handle PortableValue as a wrapper type
• Fixed PortableValue deserialization caching bugs that caused corruption and exceptions
• Added comprehensive test coverage for PortableValue round-trip scenarios

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
StateManagerTests.cs	Added comprehensive tests for PortableValue state persistence scenarios
PortableValueTests.cs	New test file covering PortableValue serialization round-trips and delayed deserialization
JsonSerializationTests.cs	Made helper methods internal for reuse in new tests
PortableValue.cs	Fixed deserialization caching logic and added robust cache management
StateScope.cs	Added special handling for PortableValue wrapper behavior
StateManager.cs	Enhanced to prevent object type reads and handle PortableValue wrapping

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[lokitoth]

PR failed to merge because IT fails due to breaking change on .ReadStateAsync<object>(). Will remove the check and re-add later.