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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