Re-land the Kotlin function arity rework (#1278) on main

[apoupard]

Re-lands the Kotlin function arity rework from spring-cloud#1278 on top of current main.

Why

spring-cloud#1278 was "squashed and merged" upstream as 62d73bf9b on 2025-06-20, but that commit sits on a line of history that was later dropped from main: no upstream branch (main, 5.0.x, 4.3.x) and no release (v4.3.1–v4.3.3, v5.0.0–v5.0.4) contains it. Upstream still detects Kotlin lambdas with KotlinLambdaToFunctionAutoConfiguration's exact-arity check, which is why F2 has to carry the arity patch on its vendored copy (fixers/5.0.4, #7). With this rework in, that patch can go.

Commits

1. The six original Fixes: Enhance Kotlin Support for Suppliers, Functions, and Consumers #1277 spring-cloud/spring-cloud-function#1278 commits, cherry-picked unchanged (authors and dates kept): 029fc7444 arity tests, a8b0f891f arity handling, 5c917a584 unit tests, 76537c10a docs, 6dfc8ee6a TypeUtils → KotlinUtils, 50b8929fb FunctionUtils → KotlinUtils. Same 60 files and identical content as the upstream squash 62d73bf9b. Only conflict (in the arity-handling commit): KotlinLambdaToFunctionAutoConfiguration is deleted by the rework but was edited on main since (Function post processing doesn't work with Kotlin classes spring-cloud/spring-cloud-function#1300 post-processing, license header) → resolved as delete, Function post processing doesn't work with Kotlin classes spring-cloud/spring-cloud-function#1300 ported in the last commit.
2. b30982b87: adaptations to current main
   • Function post processing doesn't work with Kotlin classes spring-cloud/spring-cloud-function#1300 post-processing: KotlinFunctionWrapper.getKotlinLambdaTarget(), implemented by all 15 Kotlin wrappers; SimpleFunctionRegistry and ContextFunctionCatalogAutoConfigurationKotlinTests use it instead of the removed KotlinLambdaToFunctionAutoConfiguration.KotlinFunctionWrapper.
   • Flow input/output types: the non-suspend Flow wrappers (FlowToPlain, FlowToFlow, ConsumerFlow, SupplierFlow) declared Flux<Flow<T>>; they now declare Flux<T> via KotlinUtils.getFlowTypeArguments, like the suspend wrappers. In June a failed element conversion silently fell back to the raw value; with main's stricter JSON conversion (fix(context): propagate JSON deserialization error instead of silent ClassCastException spring-cloud/spring-cloud-function#1429) it surfaced as 500 / wrong body in KotlinFunctionArityBeanTest.testFunctionFlowToPlain / testFunctionFlowToFlow.
   • Reactor nullness: CoroutinesUtils generic outputs bounded to Any (Flux<T : Any>).
   • Boot 4: AutoConfigureWebTestClient moved to org.springframework.boot.webtestclient.autoconfigure.
   • Checkstyle: 20xx-present license headers on the new files, import order.

Verification

./mvnw -pl spring-cloud-function-context,spring-cloud-function-kotlin -am test, all green, no flakes:

module	tests
spring-cloud-function-context	232 (13 skipped)
spring-cloud-function-web	199 (25 skipped)
spring-cloud-function-adapter-aws	55
spring-cloud-function-kotlin	368 (incl. the arity suites from spring-cloud#1278)

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