fix: restore Python SDK compatibility for conformance and samples

[adrianmcphee]

Summary

• fix schema initialization order by defining Version, ReverseDomainName, and Entity before dependent models
• make Version and ReverseDomainName hashable (frozen=True) for dict-key use
• normalize discovery-profile parsing in Pydantic v2 to accept both legacy and keyed/generated payload shapes
• restore legacy module compatibility for conformance and samples (*_req / *_resp aliases)
• add AP2 compatibility aliases used by clients (Ap2CompleteRequest, CheckoutResponseWithAp2)

Validation

• conformance module import smoke test passes (14/14) with editable-installed SDK
• ProtocolTest.test_discovery passes against a local mock discovery server
• IntegrationTestBase smoke path passes (service endpoint resolution + checkout payload construction)
• discovery parsing verified for both legacy and keyed payload formats

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[damaz91]

@adrianmcphee we are working on updating the SDK, most of the issues you are noting here should be fixed in #16 - as for retro-compatibility with samples and conformance, we'll need to update those as well