Shared primitive checkpoint encoding contract coverage across Python and .NET

[davidahmann]

Opened issue #4706 and PR #4707 to pin down the narrow checkpoint contract both runtimes already support: primitive JSON values, arrays, and objects should round-trip without shape drift. The branch adds one shared fixture plus one Python test and one .NET test. Python validation passed with cd python && uv run pytest packages/core/tests/workflow/test_checkpoint_encoding_contract.py; the .NET test is blocked locally because this machine has .NET SDK 9.0.109 while the repo targets net10.0.

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