Add additional server unit tests

[fabricekabongo]

Summary

• add a mock listener handler to exercise server start and stop lifecycle
• add coverage for error conditions when creating listeners

Testing

• go test ./server

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Codex Task

[deepsource-io]

Here's the code health analysis summary for commits a8cfbf4..8f44ed2. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Docker	✅ Success		View Check ↗
Secrets	✅ Success		View Check ↗
Test coverage	✅ Success		View Check ↗
Go	✅ Success		View Check ↗

Code Coverage Report

Metric	Aggregate	Go
Composite Coverage	68% (up 4% from main)	68% (up 4% from main)
Line Coverage	68% (up 4% from main)	68% (up 4% from main)

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