[BUG] Data Prepper allows creating pipelines with multiple sources

Describe the bug

Data Prepper only supports a single source. But, the configuration will pass if multiple are provided. It appears that the first is chose.

To Reproduce
Steps to reproduce the behavior:

1. Create a pipeline like this:

pipeline:
  workers: 2
  source:
    http:
    otel_trace:
  sink:
    - stdout:

2. Run Data Prepper

See that the http source starts.

2025-05-20T19:17:13,762 [pipeline-sink-worker-2-thread-1] INFO  org.opensearch.dataprepper.plugins.source.loghttp.HTTPSource - Started http source on port 2021...

Expected behavior

Data Prepper should fail indicating that only one source may be defined.

Screenshots
N/A

Environment (please complete the following information):

Data Prepper 2.11.0

Additional context

N/A