docs(mongodb): Add @ReflectiveAccess to Fruit POJO and fix native-image documentation

[Vin-it-9]

Summary

This PR improves the MongoDB guides by fixing native-image compatibility issues and correcting outdated wording.

Changes included

1. Add @ReflectiveAccess to Fruit POJO

The MongoDB Java driver’s POJO codec uses reflection to access constructors and fields.
When building GraalVM native images, reflection metadata must be explicitly provided.

Adding @ReflectiveAccess ensures Micronaut’s Graal annotation processor generates the required reflection configuration automatically.

Files updated:

• micronaut-mongodb-synchronous/java/Fruit.java
• micronaut-mongodb-asynchronous/java/Fruit.java
• micronaut-mongodb-synchronous/kotlin/Fruit.kt

2. Documentation improvements

• Added a native-image note explaining why @ReflectiveAccess is required for MongoDB POJO codec.
• Corrected “mongo-sync” → “mongo-reactive” in the asynchronous guide’s Test section.
• Cleaned formatting around the POJO section so examples render properly.

Files updated:

• micronaut-mongodb-synchronous.adoc
• micronaut-mongodb-asynchronous.adoc

Why this is needed

Without reflection metadata, GraalVM native images fail with:

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