SOLR-18089: Configure zookeeper.maxCnxns for embeddedZkServer

[openworld-maker]

What

Fixes SOLR-18089 by explicitly configuring ZooKeeper's zookeeper.maxCnxns system property to 0 for embedded ZooKeeper startup when it is not already set.

Why

Embedded ZK startup emits:
"maxCnxns is not configured, using default value 0."

ZooKeeper uses zookeeper.maxCnxns with default 0. Setting it explicitly suppresses the warning while preserving behavior.

Changes

• Added constants in SolrZkServer:
  • ZK_MAX_CNXNS_PROPERTY = "zookeeper.maxCnxns"
  • ZK_MAX_CNXNS_DEFAULT = "0"
• Added ensureZkMaxCnxnsConfigured() and invoked it in:
  • SolrZkServer.start() before ZooKeeper startup
  • ZkTestServer.run(...) before server initialization in test framework
• Guarded behavior: only sets property when currently unset (does not override user-provided value).

Tests

• Added ZkTestServerTest#testEmbeddedZkServerSetsMaxCnxnsDefaultWhenUnset:
  • clears zookeeper.maxCnxns
  • starts embedded ZkTestServer
  • asserts property equals "0"
  • restores prior property value in finally

Verification

• :solr:test-framework:test --tests org.apache.solr.cloud.ZkTestServerTest passed.
• Full tidy check was attempted but hit unrelated flaky failures in solrj tests (CloudSolrClientCacheTest).
• Ran equivalent focused validation for touched modules:
  • tidy
  • :solr:core:check -x test
  • :solr:test-framework:check -x test