Fix Feign JSON converter to use application JsonMapper modules

[arimu1]

Motivation and Context

Fixes #1376 — Feign could not deserialize types that need Jackson modules registered as beans (e.g. JavaxMoneyModule / MonetaryAmount), while the same app's @RestController worked.

FeignHttpMessageConverters called registerDefaults(), which can install a classpath-default JacksonJsonHttpMessageConverter whose mapper does not include application JacksonModule beans. Boot's ClientHttpMessageConvertersCustomizer path is not always enough for Feign child contexts; the proven workaround was a late HttpMessageConverterCustomizer that reinstalls a converter backed by the application JsonMapper.

How has this been tested?

• Unit: FeignHttpMessageConvertersTests (concurrent init + application JsonMapper / module coverage) — 2/2 (Temurin 21)
• ./mvnw -pl spring-cloud-openfeign-core test -Dtest=FeignHttpMessageConvertersTests -Pspring

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

Changes

• After applying ClientHttpMessageConvertersCustomizers, apply the application JsonMapper (when present) via withJsonConverter, so Feign uses the same modules as MVC.
• Resolve JsonMapper via BeanFactory without requiring Jackson on the configuration class classpath.
• Regression test with a synthetic abstract type + JacksonModule (stand-in for money types).

[ryanjbaxter]

See my comment here #1376 (comment)

[arimu1]

@ryanjbaxter Thanks for the pointer to #1306 / Spring Cloud 2025.0.1 (OpenFeign 5.0.1).

I'll treat this as likely already fixed on the ClientHttpMessageConvertersCustomizer / Boot 4 path for the general Jackson config case. For #1376 specifically (application JacksonModule beans such as JavaxMoneyModule not applied to Feign's JSON converter), could @a86rbchn re-test with Spring Cloud 2025.0.2 and confirm whether MonetaryAmount still fails to deserialize?

If it still fails on 2025.0.2, this PR's explicit application-JsonMapper wiring remains relevant; if it works, we can close #1399 / #1376 as fixed upstream. Happy to close from our side once confirmed.

[ryanjbaxter]

I am going to close this for now and we can reopen it if necessary