quarkus: Arc: Regression issue when mapping `@ConfigMapping` using a custom ConfigSource implementation

Describe the bug

This issue affects all users that provide a custom config source (org.eclipse.microprofile.config.spi.ConfigSource). For example:

public class CustomConfigSource implements ConfigSource {

    private static final int ORDINAL = 999;
    private static final String PROPERTIES_FILE = "/configsource.properties";

    // ...
}

Where configsource.properties is:

from.custom.source.person.name=From Config Source
from.custom.source.person.age=33

When mapping:

@Path("/config-mapping")
public class ConfigMappingResource {

    @Inject
    @ConfigMapping(prefix = "from.custom.source.person")
    PersonInterface personFromCustomSourceInterface;

It does not work any longer by failing with the following exception:

[app] Caused by: io.smallrye.config.ConfigValidationException: Configuration validation failed:
08:56:23.182 INFO  [app]        java.util.NoSuchElementException: SRCFG00014: The config property from.custom.source.person.name is required but it could not be found in any config source
08:56:23.183 INFO  [app]        java.util.NoSuchElementException: SRCFG00014: The config property from.custom.source.person.age is required but it could not be found in any config source
08:56:23.184 INFO  [app]        at io.smallrye.config.ConfigMappingProvider.mapConfiguration(ConfigMappingProvider.java:840)
08:56:23.185 INFO  [app]        at io.smallrye.config.ConfigMappingProvider.mapConfiguration(ConfigMappingProvider.java:796)
08:56:23.186 INFO  [app]        at io.smallrye.config.SmallRyeConfigBuilder.build(SmallRyeConfigBuilder.java:403)
08:56:23.186 INFO  [app]        ... 17 more

The root cause of this issue is this commit https://github.com/quarkusio/quarkus/commit/fd4b3028afbfff649c6e92e8bb51a65a47fa6bef. Note that this is a regression test that was working fine in 2.1.2.Final.

The issue also affects the extension quarkus-spring-cloud-config-client to inject properties using @ConfigMapping.

Expected behavior

No response

Actual behavior

No response

How to Reproduce?

1. git clone https://github.com/Sgitario/quarkus-test-suite
2. cd quarkus-test-suite
3. git checkout reproducer_19448

Then, we can reproduce it using a custom config source directly: 4.a cd properties 5.a mvn clean verify -Dit.test=ConfigMappingResourceIT

Or using the spring cloud config extension: 4.b cd spring/spring-cloud-config 5.b mvn clean verify

All the tests should work, but they are failing.

Output of uname -a or ver

No response

Output of java -version

No response

GraalVM version (if different from Java)

No response

Quarkus version or git rev

999-SNAPSHOT

Build tool (ie. output of mvnw --version or gradlew --version)

No response

Additional information

No response