refactor: remove deprecated curator ServiceAnnouncer stuff

[clintropolis]

Description

This PR removes the org.apache.druid.curator.discovery.ServiceAnnouncer, and is the third in a series of ZooKeeper cleanup PRs following #19377 and #19500. This ServiceAnnouncer/CuratorServiceAnnouncer stuff (not to be confused with org.apache.druid.curator.announcement.ServiceAnnouncer) used the curator-x-discovery library to write service announcements, but nothing in production paths was reading them:

• leader discovery uses DruidLeaderSelector (curator leader latch) + the /v1/leader HTTP endpoint, not this stuff
• task-to-task communication uses TaskLocation (host:port reported via UpdateLocationAction) + the X-Druid-Task-Id header, not this stuff
• Node-level discovery runs through DruidNodeAnnouncer + DruidNodeDiscoveryProvider, not this stuff

The only consumer of ServiceDiscovery.queryForInstances() in the repo was ServiceAnnouncerTest, verifying its own writes.

The ChatHandlerProvider interface has been simplified, as the 3-arg register(key, handler, boolean announce) method was only meaningful when there was an external announce side, and the interface has been collapsed into just a class ChatHandlerProvider since without the production ServiceAnnouncingChatHandlerProvider doing a bunch of curator stuff it is just a simple registry, making the no-op implementation unecessary.

Release note

Removed deprecated legacy ZooKeeper service announcement that was using the curator-x-discovery library. The library dependency has also been removed. The following configs have been removed and will be ignored:

• druid.discovery.curator.path
• druid.discovery.curator.useDiscovery
• druid.indexer.task.chathandler.type
• druid.indexer.chathandler.publishDiscovery

[FrankChen021]

I have reviewed the code for correctness, edge cases, concurrency, and integration risks; no issues found.

Reviewed 52 of 52 changed files.

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This is an automated review by Codex GPT-5.5