Go SDK: reject a task function whose last result type is not error - #74077

This PR is related to Reject invalid registrations at registration time, which #73549 addressed. That item lists a wrong return shape among the signature errors that should fail at registration. Registration still accepted one kind of signature that makes every run of the task fail.

Why

A task function can declare its last result as a concrete type that implements error like func(airflow.Context) (int, *MyErr). Registration accepts the function. When the task runs, the Go SDK reads the last result into an error value. A nil *MyErr then becomes a non-nil error. So the task fails on every run, even when the function returns nil.

How

Change internal/bundle to require the last result of a task function to have the type error itself.

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

• Yes - Claude Code

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