[backport camel-4.14.x] CAMEL-23584: camel-kafka - align Exchange header constant names with Camel naming convention

[oscerd]

Backport of #23602 (CAMEL-23584) to the camel-4.14.x maintenance branch.

Summary

Renames the Exchange header string values in KafkaConstants from the non-Camel-prefixed kafka.* namespace to the project-wide CamelKafka* PascalCase convention, completing the camel-kafka sub-task under the CAMEL-23577 umbrella header-naming-convention sweep.

Constant	Previous value	New value
PARTITION_KEY	kafka.PARTITION_KEY	CamelKafkaPartitionKey
PARTITION	kafka.PARTITION	CamelKafkaPartition
KEY	kafka.KEY	CamelKafkaKey
TOPIC	kafka.TOPIC	CamelKafkaTopic
OVERRIDE_TOPIC	kafka.OVERRIDE_TOPIC	CamelKafkaOverrideTopic
OFFSET	kafka.OFFSET	CamelKafkaOffset
HEADERS	kafka.HEADERS	CamelKafkaHeaders
LAST_RECORD_BEFORE_COMMIT	kafka.LAST_RECORD_BEFORE_COMMIT	CamelKafkaLastRecordBeforeCommit
LAST_POLL_RECORD	kafka.LAST_POLL_RECORD	CamelKafkaLastPollRecord
TIMESTAMP	kafka.TIMESTAMP	CamelKafkaTimestamp
OVERRIDE_TIMESTAMP	kafka.OVERRIDE_TIMESTAMP	CamelKafkaOverrideTimestamp
KAFKA_RECORD_META	kafka.RECORD_META	CamelKafkaRecordMeta

Backwards-compatibility

• Java field names are unchanged → routes using the symbolic constants are unaffected.
• Routes hard-coding the literal kafka.* header strings must move to the new CamelKafka* values.
• The Endpoint DSL header accessor method names (kafkaOverrideTopic(), etc.) are unchanged; only the returned string value reflects the new convention.

Notes for this backport

• KafkaConstants on camel-4.14.x differs from main (no important=true metadata flags, different description wording). Only the string values were changed here — the 4.14.x structure is otherwise preserved.
• The version-specific upgrade-guide entry is added on the main branch (per the backport upgrade-guide policy), not on this maintenance branch.
• Keeps the bundled transform classes, KafkaHeaderDeserializer, and the camel-tracing / camel-telemetry KafkaSpanDecorator copies in sync. Regenerates the catalog and endpoint DSL artifacts.
• The camel-opentelemetry2 SpanKindTest / mock producer, the core ImportantHeaderUtils, and the important-headers.json catalog do not exist on this branch and are intentionally not part of this backport.

Test plan

• camel-kafka unit tests (KafkaProducerTest, RegexRouterTest, transform tests, KafkaHeaderDeserializerTest) — pass
• Full reactor mvn clean install -DskipTests from root — passes, all downstream generators regenerate cleanly

Reported by Claude Code on behalf of Andrea Cosentino