feat(grpc_datasource): add ConnectRPC transport for gRPC datasource

[fengyuwusong]

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Summary

This PR introduces an RPCTransport abstraction inside the gRPC data source so that subgraph calls can be made over either native gRPC or ConnectRPC. It then adds the Connect implementation, regenerates the grpctest fixture with buf so the same backing MockService can be served over both transports, and locks in end-to-end coverage through the data source layer.

ConnectRPC is the right transport whenever end-to-end HTTP/2 to the subgraph is unavailable. The feature is consumed by the matching cosmo router work (see "Related").

Related

• Required by: feat(router): add ConnectRPC transport for gRPC subgraphs cosmo#2808 (router-side grpc_protocol configuration that picks the transport per subgraph).
• Replaces: feat(grpc_datasource): add ConnectRPC transport support via RPCTransport abstraction #1453.

What's changed

Data source transport abstraction

• New RPCTransport interface (transport.go) wraps the previous direct grpc.ClientConnInterface use site so the data source can dispatch through any concrete transport.
• NewDataSource(transport, config) accepts the abstraction directly; NewDataSourceGRPC(conn, config) is a thin convenience wrapper that preserves the existing call shape for gRPC consumers.

ConnectRPC transport

• transport_connect.go implements the Connect protocol over HTTP/1.1 with both Protobuf and JSON encodings.
• Follow-up fixes from earlier reviews are folded in:
  • base64-encode binary metadata headers,
  • nil-guard the response body and cap it at 10 MB,
  • detect oversized responses and truncate the surfaced error body.
• transport_connect_test.go covers Protobuf, JSON, Connect-protocol errors, non-JSON errors, and ASCII / binary header forwarding.

grpctest fixture migration to buf

• New buf.yaml and buf.gen.yaml drive protoc-gen-go, protoc-gen-go-grpc, and protoc-gen-connect-go from a single generation step. The Mproduct.proto= plugin option remaps the proto's go_package onto the actual repository path so the generated Connect package can import productv1 cleanly.
• Makefile generate-proto target now invokes buf generate.
• productv1/productv1connect/product.connect.go is checked in.

Connect-compatible MockService

• pkg/grpctest/mockservice_connect.go: mechanically generated MockServiceConnect adapter that forwards every Connect call to the existing MockService (one method per RPC, 94 in total). The same backing implementation can now serve Connect, gRPC, and gRPC-Web from a single HTTP handler.

Data source end-to-end coverage

• pkg/engine/datasource/grpc_datasource/grpc_datasource_connect_test.go: drives the data source through the new Connect transport against the Connect-wrapped MockService for both Protobuf and JSON encodings.
• A small fixup commit migrates two test sites that landed on master between rebase points (calling NewDataSource(conn, …)) onto NewDataSourceGRPC(conn, …).

Testing

• pkg/engine/datasource/grpc_datasource: 456 tests pass (Connect cases added; existing gRPC / federation / cost / lookup tests unchanged).
• pkg/grpctest: package builds with both protoc and buf-generated outputs in sync.

Backward compatibility

• The RPCTransport introduction keeps NewDataSourceGRPC as the drop-in replacement for the old NewDataSource(conn, …) call shape; existing gRPC consumers do not need to touch anything else.
• The Connect handler/client live in a new productv1/productv1connect sub-package so they don't affect existing imports of productv1.

Checklist

• I have discussed my proposed changes in an issue and have received approval to proceed.
• I have followed the coding standards of the project.
• Tests or benchmarks have been added or updated.
• Documentation has been updated on https://github.com/wundergraph/docs-website — N/A: router-side documentation for the consuming feature lives in the matching cosmo PR (docs-website/router/...).
• I have read the Contributors Guide.

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[fengyuwusong]

CC @biubiubiuboomboomboom