Python: fix(core): allow checkpoint deserialization for orchestrations

[Manohar Paturi (ManoharPaturi)]

Motivation & Context

When workflows using orchestrations request types such as HandoffAgentUserRequest or MagenticPlanReviewRequest are checkpointed and resumed under restricted decoding, deserialization fails with an error stating that deserialization is blocked for the type.

This occurs because _RestrictedUnpickler in _checkpoint_encoding.py checks whether the module begins with _FRAMEWORK_MODULE_PREFIX, which is set to "agent_framework.". That string matches the core package but rejects the sibling package agent_framework_orchestrations.

Description & Review Guide

• What are the major changes?
  Replaced _FRAMEWORK_MODULE_PREFIX with a tuple _FRAMEWORK_MODULE_PREFIXES containing both "agent_framework." and "agent_framework_orchestrations.". Updated _is_allowed_type and find_class to use startswith with the tuple. Updated the docstrings in _checkpoint_encoding.py and _checkpoint.py to reflect that orchestrations internal types are included in the default safe set. Added a regression test validating that pending orchestration requests roundtrip through restricted decoding.

• What is the impact of these changes?
  Workflows hosted as agents or persisted with checkpoint storage can now deserialize pending handoff and plan review requests without requiring callers to manually register those types.

• What do you want reviewers to focus on?
  Verification that the tuple prefix check in find_class and _is_allowed_type correctly allows the orchestration types while preserving existing safety restrictions against arbitrary nested dotted names.

Related Issue

Fixes #7618

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

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