SOLR-18062: Support arbitrary Kafka properties via env and sysprops

[openworld-maker]

Description

This PR completes SOLR-18062 by hardening and clarifying CrossDC Kafka pass-through property behavior.

Jira: https://issues.apache.org/jira/browse/SOLR-18062

main already included the core SOLR-18062 support for arbitrary Kafka properties via:

• SOLR_CROSSDC_KAFKA_* environment variables
• solr.crossdc.kafka.* system properties

This change focuses on missing edge-case handling, precedence clarity, and testability.

What Changed

1) Hardened pass-through extraction in ConfUtil

• Added a testable helper:
  • addAdditionalKafkaProperties(Map<String,Object> properties, Map<String,String> env, Properties sysProps)
• Preserved existing explicit config values already present in properties.
• Added explicit precedence for pass-through collisions:
  • solr.crossdc.kafka.* system properties override SOLR_CROSSDC_KAFKA_* env values for the same normalized Kafka key.
• Added edge-case filtering:
  • ignore blank / empty values
  • ignore empty normalized keys (for example bare prefixes)

2) Added focused unit coverage in ConfUtilTest

• env mapping (SOLR_CROSSDC_KAFKA_FOO_BAR -> foo.bar)
• sysprop mapping (solr.crossdc.kafka.foo.bar -> foo.bar)
• precedence for env vs sysprop conflicts
• explicit-property preservation (pass-through does not overwrite explicit value)
• blank value and empty key filtering
• verified pass-through values are preserved through KafkaCrossDcConf.getAdditionalProperties()

3) Updated CrossDC documentation

• Clarified key normalization behavior
• Documented precedence (sysprop > env for same pass-through key)
• Documented explicit setting preservation and blank-value/empty-key ignoring

Behavior / Compatibility

• Backward compatible with current SOLR-18062 prefixes and behavior.
• No changes to required CrossDC properties.
• No change to producer/consumer wiring (getAdditionalProperties() use remains as-is).

Validation

Ran locally:

• ./gradlew :solr:modules:cross-dc:test --tests org.apache.solr.crossdc.common.ConfUtilTest
• ./gradlew tidy :solr:modules:cross-dc:check -x test :solr:solr-ref-guide:check -x test