FindStyles: honor custom @JsonSerialize so ImportLayoutStyle.layout isn't dropped

[Jenson3210]

Summary

• FindStyles was dumping each Style via SnakeYAML with BeanAccess.FIELD, which bypassed any class-level @JsonSerialize. For ImportLayoutStyle that meant the Block instances (sealed interface, private final fields, no setters) could not be introspected and the recipe reported empty layout: [] / packagesToFold: [] even when the merged style was fully populated — exactly the field people most often want to verify.
• Each Style is now routed through ObjectMappers.propertyBasedMapper(...).convertValue(style, Map.class) first, so any custom serializer (e.g. ImportLayoutStyle.Serializer, which renders Blocks as readable strings like import java.*, <blank line>, import all other imports) is invoked before SnakeYAML sees the value. The BeanAccess.FIELD setup and the class-tag-suppressing Representer are no longer needed and have been removed.
• Added an importLayoutStyleSerializesBlocksAsStrings regression test that builds an ImportLayoutStyle via Builder with a real layout + packagesToFold and asserts the output contains the readable Block strings and not layout: [] / packagesToFold: []. The previous tests only exercised trivial POJO styles and would never have caught this.

Before

- org.openrewrite.java.style.ImportLayoutStyle:
    classCountToUseStarImport: 9999
    layout: [
      ]
    nameCountToUseStarImport: 9999
    packagesToFold: [
      ]

After

- org.openrewrite.java.style.ImportLayoutStyle:
    classCountToUseStarImport: 9999
    nameCountToUseStarImport: 9999
    layout:
    - import java.*
    - <blank line>
    - import all other imports
    - <blank line>
    - import static all other imports
    packagesToFold:
    - import java.util.*

The fix is general: any Style with a custom @JsonSerialize benefits, not just ImportLayoutStyle.

Test plan

• ./gradlew :rewrite-test:test --tests "org.openrewrite.FindStylesTest" (incl. new regression test)
• ./gradlew :rewrite-core:test :rewrite-test:test
• ./gradlew :rewrite-java:test --tests "*ImportLayoutStyleTest*" to confirm Serializer/Deserializer round-trip is untouched
• ./gradlew :rewrite-core:licenseFormat :rewrite-test:licenseFormat

[Jenson3210]

@steve-aom-elliott @MBoegers tagged you as you both have people interested in styles