fix(sdk): lazily load optional gRPC transport

[simonyang08]

Summary

• Keep non-gRPC SDK transports importable without the optional grpcio dependency.
• Load gRPC exports only when callers explicitly request them.
• Preserve the existing installation guidance when gRPC is explicitly selected.

This PR addresses the current SDK optional-dependency contract on develop. Issue #563 described an obsolete Docker image path and is not claimed as the active bug basis.

Validation

• Current develop baseline reproduces an import failure without grpcio.
• Focused optional-gRPC regression passes after the change.
• Explicit gRPC import still returns the existing pip install openagents[sdk] guidance.
• Ruff passes for the changed transport module and regression test.
• compileall passes for all changed files.

AI disclosure

This change was prepared with OpenAI Codex assistance and reviewed and validated locally by the contributor.

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[zomux]

Closing this draft as part of clearing the queue; it has been sitting in draft state without updates since Sep 2. To be clear, the underlying problem is real and still present on develop: grpcio is only an optional extra in pyproject, yet sdk/openagents/sdk/transports/init.py imports the gRPC transport eagerly, so the non-gRPC transports are unimportable without it. If you'd like to carry this forward, please open a fresh non-draft PR rebased on current develop with the lazy-export approach from this branch (the getattr shim plus the direct .transports.grpc imports in topology.py) and we'll review it promptly. Thanks for the thorough validation notes here.