MCR-3650 Refactor MCRConfigurableInstanceHelper into multiple classes

[toKrause]

Link to jira.

This PR only refactors, further simplification & clean-up is done in #2922.

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[sebhofmann]

Found by AI:

1. Duplicated word in error message — MCRInstantiator.java:236-237: "...has " + " has no singleton..." produces "...has has no...".
2. Unbalanced apostrophe — MCRInstantiator.java:107-108 and MCRInstantiatorUtils.java:90-91: "Instance of class " + name + "', configured in ..." has a closing ' without an opening one.
3. Mutable public sets — Options.NONE / Options.IMPLICIT in MCRInstanceConfiguration and MCRTargetType.ALL are raw EnumSets exposed as public static final. Any caller can mutate them. Wrap with Collections.unmodifiableSet(...) or use
   Set.of(...).
4. Wrong Javadoc references:
   - MCRFieldInjectable.java:30 says "is a MCRFieldInjectable" (self-referential).
   - MCRMethodInjectable.java:30 says "is a MCRFieldInjectable" (copy-paste error).
   - MCRRawPropertiesSource.java:32 says "interprets a MCRSourceBase" (should be MCRRawProperties).

[sebhofmann]

• Consider merging MCRInjectable and MCRTarget
• MCRInjectable should be sealed

[toKrause]

MCRInjectable represents a component that can, in principle, be used for injection (if it is annotated with one of the supported annotations). MCRTarget represents a component that is, in fact, being used for injection. I've fixed the class comments.

[sebhofmann]

• MCRSource should be sealed anly only permit the classis in the source package. MCRSourceType enum is a closed list so extension is not intended.

[sebhofmann]

• Consider renaming to MCRInstaceSourceBase, because getInstance is only used in MCRInstanceSource, MCRInstanceMapSource, MCRInstanceListSource
• allowedTargetTypes() is dead code, because no subclass overrides it

[toKrause]

MCRPostConstructionSource should indeed have a differing implementation of allowedTargetTypes, since it is only allowed on methods. Small bug that we can fix in this PR.