feat(antd-ruby): gRPC external-signer prepare/finalize parity with REST

[Nic-dorman]

Summary

Adds 7 new methods on Antd::GrpcClient mirroring the existing REST Antd::Client external-signer surface. Consumers can now swap Antd::Client for Antd::GrpcClient without code change in the prepare/finalize flow.

Depends on #140 (antd-side UploadService + chunks prepare/finalize).

Methods

method	purpose
prepare_upload(path, visibility:)	external-signer prepare for a file
prepare_upload_public(path)	convenience wrapper for visibility: "public"
prepare_data_upload(data, visibility:)	external-signer prepare for in-memory bytes
finalize_upload(upload_id, tx_hashes)	finalize wave-batch upload
finalize_merkle_upload(upload_id, winner_pool_hash, store_data_map:)	finalize merkle-batch upload
prepare_chunk_upload(data)	single-chunk external-signer prepare
finalize_chunk_upload(upload_id, tx_hashes)	single-chunk finalize, returns address

Proto regen

grpc_tools_ruby_protoc -I../antd/proto --ruby_out=lib --grpc_out=lib antd/v1/{common,health,data,chunks,files,upload}.proto:

• New upload_pb.rb / upload_services_pb.rb.
• chunks_pb.rb / chunks_services_pb.rb gain PrepareChunk / FinalizeChunk.
• common_pb.rb adds PaymentEntry.

Implementation notes

• New @upload_stub = Antd::V1::UploadService::Stub.new(...) field on GrpcClient.
• Private build_prepare_upload_result(resp) helper maps PrepareUploadResponse proto into the existing PrepareUploadResult struct, populating merkle-only fields (depth, pool_commitments, merkle_payment_timestamp) only when payment_type == "merkle".

Tests

test/test_grpc_client.rb:

• FakeGrpc::ChunkStub gains prepare_chunk (new + already-stored short-circuit) and finalize_chunk (echoes upload_id into address).
• New FakeGrpc::UploadStub covers prepare_file_upload / prepare_data_upload / finalize_upload with visibility round-trip via upload_id encoding and merkle vs wave-batch branches.
• build_fake_client / build_error_client updated to inject @upload_stub.
• 12 new test cases parallel to the antd-rust / antd-go / antd-py / antd-java / antd-kotlin / antd-csharp suites.

Gates on dev2

• All 12 new tests pass
• Pre-existing baseline has 4 failures in the cost mocks (test_data_cost, test_file_cost, etc.) where the fake cost stub returns a partial OpenStruct; unrelated to this PR (verified by running tests on the parent commit).