Support Taskflow-like syntax in the Go SDK

[jason810496]

Background

Today a Go task function takes ctx, an sdk.Client / sdk.VariableClient,
and a *slog.Logger, and pulls XCom data explicitly:

// go-sdk/example/bundle/main.go
func transform(ctx context.Context, client sdk.VariableClient, log *slog.Logger) error {
    val, err := client.GetVariable(ctx, "my_variable")
    ...
}

Java has just landed Taskflow-style task signatures under #66332, where
the upstream task's return value is declaratively injected into the
downstream task's parameter list rather than pulled via the client. We
want the same shape in Go so authors can write:

func extract(ctx context.Context) (SomeType, error) { ... }
func transform(ctx context.Context, upstream SomeType) error { ... }

What needs to happen

1. Decide how to declare task-to-parameter wiring in Registry.AddTask
   (does depends []string carry positional binding, or do we use typed
   task handles returned by AddTask?).
2. Use reflection in bundlev1.NewTaskFunction to recognise an extra
   typed parameter and treat it as an XCom pull from the named upstream.
3. Serialise the binding into the Dag JSON in coordinator-mode so the
   supervisor knows which XCom to feed in.
4. Update example/bundle/main.go to a Taskflow-style example.

Acceptance criteria

• A Go task can declare an upstream-typed parameter and receive the
  upstream's return value at execution time without calling
  client.GetXCom explicitly.
• Serialisation matches what the Python supervisor expects for a
  Taskflow-wired Dag (validated against test_dags.yaml).
• The existing client-pull pattern remains supported.

Context

• Java parallel: Support Taskflow in Java SDK #66332 (Done).
• @task.stub argument-passing groundwork: Make @task.stub able to pass argument #66330 (Done).
• Same area as task dependency representation: Improve Task Dependency Representation #66331.