CAMEL-23515: camel-nats - Use dedicated HeaderFilterStrategy aligned with sibling components

[oscerd]

Summary

camel-nats defaulted its headerFilterStrategy to a bare
new DefaultHeaderFilterStrategy() with no further configuration
(NatsConfiguration.java:119). The base class with no inFilter /
inFilterPattern / inFilterStartsWith configured returns false for
every header, which means every inbound NATS message header is copied
onto the Camel Exchange as-is — including Camel* / camel* /
org.apache.camel.* keys that drive component behaviour downstream.

Sibling components (camel-kafka, camel-mail, camel-coap,
camel-google-pubsub, camel-jms, camel-sjms, …) ship a dedicated subclass
that configures setInFilterStartsWith("Camel", "camel", "org.apache.camel.")
and setLowerCase(true). This PR aligns camel-nats with that
convention.

Changes

• New NatsHeaderFilterStrategy following the
  KafkaHeaderFilterStrategy / MailHeaderFilterStrategy shape.
• NatsConfiguration now defaults headerFilterStrategy to
  new NatsHeaderFilterStrategy().
• Unit test covering inbound/outbound Camel* filtering, lowercase
  variants, and pass-through of non-Camel headers.
• 4.21 upgrade-guide entry under the existing === camel-nats
  section.

Test plan

• Module build: mvn -pl components/camel-nats -DskipTests install
• New unit test: NatsHeaderFilterStrategyTest (4 assertions)
• CI: full reactor build + existing camel-nats unit/integration tests

JIRA

https://issues.apache.org/jira/browse/CAMEL-23515

Backports planned for 4.18.x and 4.14.x per fixVersions on the JIRA.

Claude Code on behalf of Andrea Cosentino

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🧪 CI tested the following changed modules:

• components/camel-nats
• docs

All tested modules (8 modules)

• Camel :: JBang :: MCP
• Camel :: JBang :: Plugin :: Route Parser
• Camel :: JBang :: Plugin :: TUI
• Camel :: JBang :: Plugin :: Validate
• Camel :: Launcher :: Container
• Camel :: Nats
• Camel :: YAML DSL :: Validator
• Camel :: YAML DSL :: Validator Maven Plugin

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