.NET: Add shared primitive checkpoint encoding contract tests

[davidahmann]

Motivation and Context

Checkpoint values that are already JSON primitives, arrays, and objects should round-trip consistently through the Python and .NET checkpoint stores. There was no shared regression fixture proving that narrow contract across both runtimes. Closes #4706.

Description

This adds a shared testdata/checkpoint_primitive_contract.json fixture plus one Python test and one .NET test that assert the same primitive payload can be stored and read back without shape drift. The scope is intentionally limited to the primitive JSON contract that both runtimes already claim to support.

Contribution Checklist

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[davidahmann]

Validation on this branch:

• cd python && uv run pytest packages/core/tests/workflow/test_checkpoint_encoding_contract.py ✅
• dotnet test dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests --filter PrimitiveCheckpointEncodingContractTests ⚠️ baseline/environment mismatch locally: installed SDK is 9.0.109, but the repo targets net10.0

Initial CI after push:

• add_label ✅
• license/cla queued (policy/account gate)
• Issue/PR: add title prefix queued

Inspired by research context: CAISI publishes independent, reproducible AI agent governance research: https://caisi.dev