engine-java: make the 'a JavaDelegate must NOT be a @Component' rule observable (#7223)

[NicoleNG18]

Cause

The rule that a BPMN JavaDelegate must not be annotated @Component was documented in four places (.claude/docs/client-java.md, engine-java/CLAUDE.md, the two assistant guides, and a scaffold stub comment) but nothing in the runtime enforced, warned about, or even noticed a violation. After #7189 the old symptom - injected fields reading null at runtime - is gone, because both the container-built singleton and the Flowable-built instance are now wired. So a developer had no signal at all: an annotated delegate silently becomes a registered bean nobody runs, and a stray candidate for every List<JavaDelegate> injection.

Change

• ComponentContainer.createUnmanaged now emits one LOGGER.warn naming the class and the rule when the delegate type is a bean, making the violation observable at the moment it happens. A unit test asserts the warn fires.
• ComponentContainerUnmanagedTest.an_unsatisfied_dependency_is_refused_and_is_not_a_rebuild_error built the container from no classes, so its wiringErrors().isEmpty() assertion was trivially true whatever createUnmanaged did; it now builds with a registered bean and a delegate needing an absent type, so the "not a rebuild error" half is load-bearing.
• DirigibleJavaCallDelegate: the fallback instantiation error no longer advises "or declare the collaborators it injects as @component" - that branch is reached only when the class declares no injection point, so the advice can never apply; reverted to the plain message the camel sibling (DirigibleJavaInvokerImpl) already uses. Its class javadoc now lists @PostConstruct alongside constructor and @Inject field as an injection point (all three are counted by declaresInjectionPoint / ClientBeanFactory).
• ComponentContainer / JavaLoader javadoc no longer list @Extension among the consumers - the SDK has no such annotation (sdk/component/ holds only Beans, Component, Inject, Repository).

The documented rule itself is correct and unchanged; this PR only makes it observable and corrects the stale text from #7189.

Verification

• mvn formatter:validate after wiping the formatter cache - BUILD SUCCESS.
• engine-java + engine-bpm-flowable unit suites - green (129 tests, including the new warn test in ComponentContainerUnmanagedTest).
• JavaDelegateInjectionIT - green (3/3); confirms both delegate paths still wire and the unsatisfiable-dependency step still dead-letters rather than failing the deployment.
• Release-profile javadoc build - green on both changed modules.

Not run: the full IT suite (unchanged runtime behaviour beyond a log line and text; CI covers it).

Fixes #7223