Clarify Task Instance terminal state reporting

[fat-catTW]

Why

The Task SDK client previously exposed this operation as finish(), but that name made it sound like the generic path for any task that stops executing. In practice, this method only reports non-success terminal Task Instance states; success, retry, defer, reschedule, and awaiting-input transitions use dedicated methods and payloads.

Change

Rename the Task SDK client method used for non-success terminal Task Instance state updates from finish() to set_terminal_state().

This better reflects that success, retry, defer, reschedule, and awaiting-input states each use dedicated API payloads and should not fall back to this generic non-success terminal-state update.

Updated the supervisor call sites, comments, unit tests, and Task SDK integration test usage.

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

• Yes (please specify the tool below)

---

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.