chore(core): remove the dead clean_handlers mechanism

[AlinsRan]

Description

Follow-up to #13717 (comment).

After #12426 moved health checks to healthcheck_manager, nothing registers a clean handler any more: config_util.add_clean_handler() has no caller outside its own unit test. That makes every fire_all_clean_handlers() call a no-op and every item.clean_handlers = {} initialization dead weight on each config item.

This removes the mechanism uniformly instead of dropping it from one config backend only:

• core.config_util: drop add_clean_handler, cancel_clean_handler, fire_all_clean_handlers
• config_etcd / config_yaml / config_xds: drop the fire sites and the clean_handlers initialization
• plugin.lua: drop the field from the dummy global rule
• control/v1.lua: drop the field from the routes/services dump
• docs: drop the stale clean_handlers entries from the control API response samples
• t/core/config_util.t: drop the test that only covered the removed API

No behavior change — the removed code paths were unreachable.

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (if not, please discuss on the APISIX mailing list first)

[Copilot]

🟢 Ready to approve

The removed APIs/fields have no remaining references in the codebase and the updates are consistently applied across runtime config backends, control output shaping, docs, and tests.

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Pull request overview

Removes the now-dead clean_handlers mechanism from APISIX core configuration objects. Since health checks were moved to healthcheck_manager, no runtime config item registers clean handlers anymore, making handler initialization and firing sites unreachable and unnecessary.

Changes:

• Removed add_clean_handler, cancel_clean_handler, and fire_all_clean_handlers from apisix.core.config_util.
• Dropped clean_handlers initialization and all fire sites from config backends (etcd/yaml/xds) and related objects.
• Updated Control API output shaping + docs samples, and removed the unit test that only covered the deleted API.

File summaries

File	Description
apisix/core/config_util.lua	Removes the clean-handler API surface from the utility module.
apisix/core/config_etcd.lua	Removes clean-handler initialization/firing in etcd config sync paths.
apisix/core/config_yaml.lua	Removes clean-handler initialization/firing in YAML config sync paths.
apisix/core/config_xds.lua	Removes clean-handler initialization/firing in xDS config sync paths.
apisix/plugin.lua	Drops clean_handlers from the dummy global rule object.
apisix/control/v1.lua	Stops explicitly stripping clean_handlers from dumped routes/services.
docs/en/latest/control-api.md	Removes clean_handlers from Control API response examples.
t/core/config_util.t	Removes the test that only exercised the deleted clean-handler API.
t/core/config_etcd.t	Updates test fixture data to no longer include clean_handlers.

Review details

• Files reviewed: 9/9 changed files
• Comments generated: 0
• Review effort level: Low

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[membphis]

LGTM