fix: keep non-gRPC transports optional

[simonyang08]

Resubmits #647 per the invitation in #647 (comment) — fresh, non-draft, and rebased onto current develop (81289018). The original analysis and approach are unchanged; thanks @zomux for confirming the problem is still live.

Problem

grpcio is only an optional extra in pyproject.toml, yet sdk/openagents/sdk/transports/__init__.py imported the gRPC transport eagerly, so none of the transports were importable without grpcio installed — including the HTTP transport that has no gRPC dependency at all.

Still reproducible on current develop (verified today): with grpc blocked from the import system, from openagents.sdk.transports import HttpTransport raises

ImportError: grpcio is required for gRPC transport. Install with: pip install openagents[sdk]

Fix (lazy exports)

• transports/__init__.py no longer imports the gRPC module up front; it exposes GRPCTransport via a module-level __getattr__ that only imports .grpc when the name is actually accessed, raising the same actionable error if grpcio is missing.
• topology.py imports .transports.grpc symbols directly where they are used, so it does not force the lazy export machinery.

Verification

• tests/test_optional_grpc.py: with grpc blocked, HttpTransport (and friends) import cleanly, grpc never lands in sys.modules, and accessing GRPCTransport raises the actionable install hint. 1 passed on this branch; on pure develop the same import raises (RED confirmed today).
• Cherry-pick applied cleanly onto develop tip 81289018; no other files touched (topology.py, transports/__init__.py, one new test).

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