[Agent Builder] Resolve overwritten default state in MCP KSC with Secret headers

[dennis-tismenko]

Summary

Fixes the usage of secret headers in the MCP Kibana Stack connectors by setting a default value for the hasHeaders field.

CleanShot.2026-01-09.at.19.02.58.mp4

Checklist

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• Unit or functional tests were updated or added to match the most common scenarios
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