GH-1391 Add single function fallback opt-out

[goutamadwant]

Summary

• Add spring.cloud.function.single-function-fallback-enabled, defaulting to true, so existing single-function lookup behavior remains unchanged.
• When disabled, an unmatched FunctionCatalog.lookup("<name>") no longer normalizes to the only registered function.
• Update configuration metadata and add regression coverage for both default and disabled lookup behavior.

Closes #1391

Testing

• ./mvnw -pl spring-cloud-function-context '-Dtest=BeanFactoryAwareFunctionRegistryTests#testSingleFunctionFallback*' test
• ./mvnw -pl spring-cloud-function-context '-Dtest=BeanFactoryAwareFunctionRegistryTests' test
• ./mvnw -pl spring-cloud-function-context test

[olegz]

Thank you for your contribution.
Next time for something like this, please add a little note to the docs.