[feat] PIP-468: Add scalable topic protocol commands and connection handling

[merlimat]

Stacked on top of #25559. This PR is opened as a draft and will be
rebased on master once #25559 lands. Reviewers: the new code in this
PR is the single commit titled "PIP-468: Add scalable topic protocol
commands and connection handling"; everything else comes from #25559.

Summary

Second PR in the PIP-468 series. Introduces the wire protocol for the
scalable-topic client/broker interaction:

• New protocol commands on PulsarApi.proto:
  CommandScalableTopicLookup, CommandScalableTopicUpdate,
  CommandScalableTopicClose, CommandScalableTopicSubscribe,
  CommandScalableTopicSubscribeResponse,
  CommandScalableTopicAssignmentUpdate.
• Commands factory methods for building each message (including the
  error variants for lookup/subscribe).
• PulsarDecoder dispatch entries for all six new message types.
• Broker-side DagWatchSession — resolves per-segment broker
  addresses, watches the metadata store for DAG changes, and pushes
  ScalableTopicUpdate frames on the wire. Tied to the transport
  connection — closed when the connection drops.
• ServerCnx handlers for ScalableTopicLookup and
  ScalableTopicClose.
• Client-side DagWatchSession interface (callback surface for the V5
  client that lands later).
• ClientCnx handler for ScalableTopicUpdate that dispatches to the
  registered session.
• ConsumerSession.sendAssignmentUpdate wired through to the new
  command sender (previously a placeholder no-op).

Tests

Three new unit-test classes:

• CommandsScalableTopicTest (8 tests) — every new factory method
  roundtrips through the full wire frame; uses
  BaseCommand.materialize() so the buffer can be safely released
  before assertions.
• ConsumerSessionTest (15 tests) — covers identity/equality,
  attach/markDisconnected, grace-timer lifecycle, all three
  sendAssignmentUpdate branches (connected, disconnected, null
  command sender), and the toProto static helper.
• DagWatchSessionTest (12 tests) — session identity, idempotent
  close, start() failure on missing topic, metadata-store listener
  registration, notification filtering (wrong path / Deleted /
  post-close), pushUpdate serializing the correct DAG to the
  connection, and post-close/no-broker-map edge cases.

The heavy NamespaceService lookup path inside start() /
buildResponse is deferred to broker integration tests — mocking the
full namespace stack here is more noise than signal.

Test plan

• ./gradlew :pulsar-common:test --tests "org.apache.pulsar.common.protocol.CommandsScalableTopicTest"
  — 8/8 pass.
• ./gradlew :pulsar-broker:test --tests "org.apache.pulsar.broker.service.scalable.*"
  — 61/61 pass (5 test classes, including the two existing ones
  from the previous PR).
• ./gradlew :pulsar-broker:checkstyleMain :pulsar-broker:checkstyleTest :pulsar-common:checkstyleMain :pulsar-common:checkstyleTest
  — all clean.

[lhotari]

LGTM, one minor question