GH-3203 Skip standalone bindings in supplierInitializer to avoid ClassCastException for output-only bindings

[seonwooj0810]

Fixes #3203

Problem

After upgrading to 5.0.2, an output-only binding (e.g. spring.cloud.stream.output-bindings=testOutput) with no backing Supplier/Function bean intermittently fails on startup with:

java.lang.ClassCastException: class ...DirectWithAttributesChannel$$SpringCGLIB$$0 cannot be cast to class java.util.function.Supplier
	at ...SimpleFunctionRegistry$FunctionInvocationWrapper.doApply(SimpleFunctionRegistry.java:826)
	...
	at ...AbstractPollingEndpoint.pollForMessage

Root cause

Commit f042adb (GH-3166) changed BindableFunctionProxyFactory.getFunctionDefinition() to always return the function definition instead of isFunctionExist() ? functionDefinition : null:

public String getFunctionDefinition() {
    //return this.isFunctionExist() ? this.functionDefinition : null;
    return this.functionDefinition;
}

For a standalone output binding the "function definition" is the binding name, so supplierInitializer now does functionCatalog.lookup("testOutput"), which resolves the plain output channel bean. isSupplier() reports true, a SourcePollingChannelAdapter is built on top of the channel, and the first poll casts the channel to Supplier → ClassCastException. In 5.0.1 getFunctionDefinition() returned null for these bindings, so lookup(null) returned null and they were skipped.

Fix

Guard the supplier initialization loop with proxyFactory.isFunctionExist(). Standalone bindings are constructed with functionExist == false (see createStandAloneBindingsIfNecessary, where new BindableFunctionProxyFactory(..., sourceFunc != null) and sourceFunc == null for a binding without a backing function), so they are now skipped — exactly the 5.0.1 behavior. Declared functions are constructed with functionExist == true, so real suppliers are unaffected. The change is local to supplierInitializer and does not revert the getFunctionDefinition() change that GH-3166 relies on for dynamic binding creation.

Test evidence

Added OutputOnlyBindingSupplierTests which starts an app with an output-only binding and asserts no SourcePollingChannelAdapter is created.

• Without the fix the test fails: Expecting empty but was: {"testOutput_spca"=bean 'testOutput_spca'} (the SPCA that triggers the CCE on poll).
• With the fix the test passes.
• Full core/spring-cloud-stream module test suite is green: Tests run: 32, Failures: 0, Errors: 0, Skipped: 1 (pre-existing skip).

Verification done: (1) no in-flight PR (gh pr list search empty); (2) no self-claim on the issue; (4) grepped main and confirmed the commented-out guard in getFunctionDefinition() and the unguarded supplierInitializer; reproduced the SourcePollingChannelAdapter creation deterministically in a test on current main.