Add typed/Pydantic models for OpenShell specs

Motivation

Constructing OpenShell specs directly from raw protobuf objects can be verbose and hard to validate. Typed Python models would improve developer experience and make configuration errors easier to understand.

Proposed change

Add typed models, potentially Pydantic-based, for common OpenShell spec construction. These models should validate user input and convert explicitly to the existing protobuf types.

Acceptance criteria

• Add typed model(s) for common sandbox/session/spec construction paths.
• Provide clear validation errors for invalid inputs.
• Keep protobuf interop explicit and backward compatible.
• Add unit tests for valid construction, invalid values, and protobuf conversion.
• Document at least one end-to-end example.

Notes

This is primarily a Python SDK developer-experience improvement and should not require gateway/proto changes.

Locate the Python SDK's existing OpenShell spec-construction paths and protobuf types first. Add unit tests covering valid construction, invalid values, and explicit protobuf conversion, then document one end-to-end example; done means validation is clear and existing protobuf interoperability remains backward compatible.