[feat] PIP-468: Add scalable topics and segments admin APIs with CLI

[merlimat]

Stacked on top of #25564 (which is stacked on #25559). Opened as a
draft; will be rebased on master once the upstream PRs land. Reviewers:
the new code in this PR is the single commit titled "PIP-468: Add
scalable topics and segments admin APIs with CLI" — everything else
comes from #25559 and #25564.

Summary

Third PR in the PIP-468 series. Wires up the admin/management surface
for scalable topics:

REST APIs (broker)

• ScalableTopics REST resource — create / list / get-metadata / get-stats /
  delete / create-subscription / delete-subscription / split / merge.
  Includes redirectToControllerLeaderIfNeeded for write operations so
  non-leader brokers automatically 307 to the leader.
• Segments REST resource — cross-broker segment-topic
  create / terminate / delete with ownership-based routing.

Client-admin API

• ScalableTopics interface on PulsarAdmin with sync and async
  counterparts for every operation.
• ScalableTopicsImpl — REST client wiring.
• Data types under org.apache.pulsar.common.policies.data:
  ScalableTopicMetadata (plus nested SegmentInfo and HashRange),
  ScalableTopicStats (plus nested SegmentStats and SubscriptionStats
  records), ScalableSubscriptionType enum.

CLI

• CmdScalableTopics pulsar-admin subcommand with list, create,
  get-metadata, stats, delete, split-segment, merge-segments.

Broker integration

• ScalableTopicController gains createSubscription,
  deleteSubscription, and getStats — exposing admin-visible
  operations on top of the infrastructure added in [feat] PIP-468: Add ScalableTopicController and broker infrastructure #25559.
• ScalableTopicService gains matching per-topic delegations +
  admin-layer entry points (createScalableTopic, deleteScalableTopic,
  splitSegment, mergeSegments, etc.) used by the REST handlers.
• Small tweak to DagWatchSession to pick up the reviewed changes from
  [feat] PIP-468: Add scalable topic protocol commands and connection handling #25564.

Tests

New test classes (24 tests)

• ScalableTopicStatsTest (10 tests) — construction (no-args /
  @Builder / all-args), @Builder.Default fresh-per-instance
  behavior, equals/hashCode, and the nested SegmentStats /
  SubscriptionStats records.
• ScalableTopicMetadataTest (11 tests) — same construction patterns,
  plus SegmentInfo.isActive / isSealed (including the unknown-state
  branch), full getters on a sealed segment, and HashRange.toString()
  format (lowercase 4-digit hex).
• ScalableSubscriptionTypeTest (3 tests) — locks down the enum values
  so a rename breaks the build; valueOf roundtrip and rejection of
  unknown names.

Expanded in this commit

• ScalableTopicControllerTest — 24 tests covering all controller
  methods including createSubscription / deleteSubscription /
  getStats / stats-after-split-and-subscriptions / etc.
• ScalableTopicServiceTest — 16 tests covering service lifecycle,
  create/delete topic, get-or-create-controller (caching + failure +
  eviction), release, delegation to split / merge / subscriptions /
  stats / register / disconnect, and topic cleanup.

Coverage on this branch

Test class	Tests
CommandsScalableTopicTest (from #25564)	8
ConsumerSessionTest (from #25564)	15
DagWatchSessionTest (from #25564)	12
ScalableTopicControllerTest	24
ScalableTopicServiceTest	16
SegmentLayoutTest (from #25559)	15
SubscriptionCoordinatorTest (from #25559)	11
ScalableTopicStatsTest	10
ScalableTopicMetadataTest	11
ScalableSubscriptionTypeTest	3
Total	125

Test plan

• ./gradlew :pulsar-client-admin-api:test --tests "org.apache.pulsar.common.policies.data.Scalable*Test" — 24/24 pass.
• ./gradlew :pulsar-broker:test --tests "org.apache.pulsar.broker.service.scalable.*" — 93/93 pass.
• ./gradlew :pulsar-common:test --tests "org.apache.pulsar.common.protocol.CommandsScalableTopicTest" — 8/8 pass.
• ./gradlew :pulsar-client-admin-api:checkstyleMain :pulsar-client-admin-api:checkstyleTest :pulsar-broker:checkstyleMain :pulsar-broker:checkstyleTest :pulsar-common:checkstyleMain :pulsar-common:checkstyleTest :pulsar-client-admin-original:checkstyleMain :pulsar-client-tools:checkstyleMain — clean.

[lhotari]

LGTM, I guess authnz will come later.