SOLR-18214: Default configset clone base to _default when omitted

[openworld-maker]

What

Fixes SOLR-18214 by making baseConfigSet optional in the v2 configset clone/create API path.

When baseConfigSet is omitted, the API now uses _default (matching documented behavior and v1 mapping semantics).

Changes

• In CloneConfigSet, resolve omitted/empty requestBody.baseConfigSet to CloneConfigsetRequestBody.DEFAULT_CONFIGSET.
• Use the resolved value consistently for existence validation and command payload construction.
• Add focused unit test for defaulting behavior:
  • CloneConfigSetAPITest.testMissingBaseConfigSetDefaultsToDefaultConfigset
• Add SolrCloud integration coverage for the real v2 request path:
  • TestConfigSetsAPI.testV2CreateWithoutBaseConfigsetUsesDefault

Validation

Ran in Docker (JDK 21):

• :solr:core:test --tests 'org.apache.solr.cloud.TestConfigSetsAPI' --tests 'org.apache.solr.handler.configsets.*'
• :solr:core:spotlessCheck

Result: BUILD SUCCESSFUL (56 tests, 1 skipped for selected test scope).