fix: freeze public testing API state contracts

[seratch]

This pull request fixes a gap in the prospective v0.21.0 released API contract for the new scripted testing utilities. The contract already covered their modules, exports, and callable signatures, but it did not protect their intended public state surfaces. As a result, supported properties or TypedDict fields could be removed or changed without failing the release-contract check.

The existing release-contract policy and generator now:

• Capture explicitly selected public properties for the scripted Model, Realtime, Sandbox, and Voice testing utilities.
• Capture selected field names, requiredness, and declared annotations for ModelStepSpec, SandboxStepSpec, and RealtimeConnectCall.
• Resolve properties exposed by the public sandbox factory without freezing its private implementation class.
• Refresh baseline_commit provenance when only a selected TypedDict surface changes.
• Report missing or incompatible selected surfaces through the existing actionable contract-diff mechanism.

The policy remains deliberately scoped: it does not freeze arbitrary class attributes, private implementation classes, callback internals, or every TypedDict in the repository. This change does not alter runtime behavior, canonical import paths, or existing testing API signatures.

[chatgpt-codex-connector]

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