Improve error message for multiple implementations in HeteroDescribableConfigurator

[somiljain2006]

Fixes #2589

JCasC actually already correctly rejects multiple implementations for a single-valued HeteroDescribable (like serverConfiguration in the OpenID Connect plugin) and fails the configuration. The underlying validation logic is already working as intended.

However, when this conflict occurs, JCasC throws a very generic IllegalArgumentException:

Single entry map expected to configure a <type>

While technically correct, the error does not identify which configuration entries caused the conflict, requiring users to inspect the configuration manually to determine the source of the problem.

This PR improves HeteroDescribableConfigurator.configureMapping to explicitly list the conflicting keys provided by the user, making configuration mistakes immediately clear.

Added a test covering a nested single-valued Describable configured with multiple implementations and verifying that the conflicting entries are reported in the exception message.

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