[jason810496]

• related: Reject invalid Go SDK task handler registrations up front #73549 (follow-up on post-merge review comments)

Why

Three findings from the review of #73549:

1. the variadic rejection was justified by a backwards description of reflect.Value.Call
2. NewTaskFunction panicked inside reflect before it could report a non-func input
3. --help ignored the writer Serve is given.

What

• Check reflect.Func in NewTaskFunction before Value.Pointer, which panics on an int, and Value.Type, which panics on an untyped nil. bundle.NewTaskFunction(3) now returns expected a func as input but was int instead of panicking.

• Correct the variadic rationale and tell the author what to write instead. binding.Analyze turns a ...T tail into one []T parameter, so supporting it would mean calling the function with CallSlice, and both spellings take a single argument, so []T stays the one spelling for that signature.

  // rejected: task function main.load is variadic;
  // declare the last parameter as []T instead of ...T
  func load(actx airflow.Context, names ...string) error
  
  func load(actx airflow.Context, names []string) error

• Print --help usage to the writer serve receives instead of stderr. Only help moves: pflag writes a deprecation warning for a flag the bundle defines while it parses, so that stays on stderr where it cannot land in the middle of the --airflow-metadata manifest.

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Was generative AI tooling used to co-author this PR?

• Yes, with help of Claude Code Opus 5 following the guidelines

[henry3260]

Thanks LGTM