* feat: add gRPC proxy support
* fix: respect channelOptions grpc.primary_user_agent over User-Agent header
* fix: remove non-standard grpc_proxy/no_grpc_proxy env var support
These are not recognized by any standard gRPC implementation. gRPC proxy
now uses the standard http_proxy/https_proxy/no_proxy variables like
grpc-core, grpc-go, and grpc-java.
* chore: add resolveGrpcProxyConfig tests and clean up grpc-client
Export resolveGrpcProxyConfig for testability and add unit tests covering
all proxy modes (off, on, system), auth encoding, protocol rejection,
bypass lists, and edge cases. Remove redundant cancelAndCloseConnection
call in startConnection (already guarded by addConnection). Document why
internal @grpc/grpc-js channel options are used for programmatic proxy.