Improve go/rpc API

[qmuntal]

• Replace interface{} -> any.
• Remove Rpc from rpc struct names. The package is already named rpc, there is no need to duplicate the "context" in the struct name. E.g. ModelRpcApi -> ModelApi.
• NewSessionRpc and NewServerRpc now allocate less.

While here, fix TestMultiClient.

[Copilot]

Pull request overview

Updates the Go RPC surface and generator to modernize types (any), simplify API names, and reduce allocations in the generated RPC wrappers by sharing a common service struct.

Changes:

• Replace generated interface{} usages with any in Go RPC types.
• Rename generated RPC API structs to drop the redundant Rpc/RpcApi suffix (e.g., ModelRpcApi → ModelApi).
• Reduce per-wrapper allocations by sharing a single underlying service struct across API groups; adjust JSON-RPC client to treat nil params as {}.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
scripts/codegen/go.ts	Updates Go RPC codegen: replaces interface{} → any, renames API types, and emits shared-service wrapper structure.
go/rpc/generated_rpc.go	Regenerated Go RPC types/wrappers reflecting any, renamed APIs, and shared-service construction.
go/internal/jsonrpc2/jsonrpc2.go	Changes request/notify param encoding so nil params serialize as {} to support no-params calls without allocating empty maps.

Comments suppressed due to low confidence (1)

go/internal/jsonrpc2/jsonrpc2.go:226

• This change introduces special handling for params == nil (sending {}), which is important behavior to lock down because it affects the wire protocol for no-params calls. There are already unit tests for this package; please add coverage that asserts Request/Notify serialize nil params as an empty object (and not null) so future refactors don’t regress this.

	var paramsData json.RawMessage
	if params == nil {
		paramsData = json.RawMessage("{}")
	} else {
		var err error
		paramsData, err = json.Marshal(params)
		if err != nil {
			return nil, fmt.Errorf("failed to marshal params: %w", err)
		}
	}