fix: Serialize playbooks manually on Python 3.12+

[m-horky]

• Card ID: CCT-538

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Complete Description of Additions/Changes:

The playbook verification relies signatures created from hashes of serialized snippets/playbooks. Python 3.12 changed the way it serializes OrderedDict objects into strings, making the verification fail, resulting in valid playbooks being rejected.

This patch mimics the old serialization logic in a tested and easy-to-debug way.