Add Apache License 2.0 headers to fluent source files

[Copilot]

The client-java-api-fluent sources JAR contained no license headers on any of its ~1,457 generated Java files because the fluent module had spotless entirely disabled (<skip>true</skip>).

Changes

• fluent/src/main/java/**/*.java — Added Apache License 2.0 header to all 1,457 generated fluent source files.
• fluent/pom.xml — Replaced the spotless <skip>true</skip> with a targeted license-header-only configuration using combine.self="override" to avoid inheriting the parent's full spotless config (Google Java Format, groovy formatter, wildcard-import checks) that would break generated code style.
• fluent-gen/generate.sh — Added mvn spotless:apply after code generation so future regenerations automatically pick up license headers.

The resulting spotless config in fluent/pom.xml:

<configuration combine.self="override">
    <java>
        <licenseHeader>
            <content>
/*
Copyright $YEAR The Kubernetes Authors.
Licensed under the Apache License, Version 2.0 ...
*/
            </content>
            <delimiter>package </delimiter>
        </licenseHeader>
    </java>
</configuration>

[linux-foundation-easycla]

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• ✅ login: brendandburns / name: Brendan Burns (54894eb, c4cfd8b)

[kubernetes-prow]

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• c4cfd8b Add Apache License 2.0 headers to fluent source files

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