Propagate non-recoverable JSON conversion failures instead of demoting them to a generic 500

[apoupard]

Summary

spring-cloudGH-1429 (ef533f3, already on main) made convertNonMessageInputIfNecessary rethrow (rather than swallow) a JSON conversion failure when the request's Content-Type is JSON (failOnJsonError), fixing a misleading ClassCastException into a real Jackson exception in the logs. That's a great fix for the log message - but the rethrown exception is still wrapped as a plain IllegalStateException by convertInputPublisherIfNecessary, indistinguishable from any other conversion failure. Two consequences:

• It still resolves to a generic 500 for any web layer that doesn't parse exception messages to guess what went wrong.
• There's no way for a downstream consumer (a web framework's exception resolver, a custom @ControllerAdvice, an app's own error handler) to recognize "this converter deliberately, definitively rejected this input" without inspecting the message/cause chain.

This PR adds NonRecoverableConversionException, a narrow MessageConversionException subtype thrown only from the failOnJsonError path, and lets it propagate unwrapped through convertInputPublisherIfNecessary instead of being folded into the generic IllegalStateException bucket.

• No new dependency: MessageConversionException already lives in spring-messaging, a non-optional dependency of spring-cloud-function-context.
• No behavior change for any other exception type or code path - only the failOnJsonError branch's exception type changes.

Known follow-up (intentionally out of scope here)

A function declared as Function<Message<T>, R> (explicit Message-wrapped type) never reaches this path at all: convertInputMessageIfNecessary deliberately returns the original, unconverted message whenever the target type is itself Message<T> (to support legitimate no-conversion-needed cases like KafkaNull), which silently masks the same class of failure a different way. Functions declared over the plain payload type - the common case, and the only shape the new PojoFunctionIntegrationTests covers - are unaffected and fixed by this change.

Test plan

• SimpleFunctionRegistryTests#testReactivePojoFunctionPropagatesNonRecoverableConversionExceptionForMalformedJson (new) - removal-proofed: fails without the fix, passes with it.
• SimpleFunctionRegistryTests#testReactiveFunctionMessages (existing, valid-JSON case) - still green, confirming no behavior change on the success path.
• Full spring-cloud-function-context + spring-cloud-function-web suite: 199 tests, 0 failures, 0 errors (including the new PojoFunctionIntegrationTests).

Was previously opened against spring-cloud#1436 by mistake and closed - this is the fork-local equivalent.

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