🧪 Add tests and fix payload dictionary validation in idstack-manifest-merge

[savvides]

🎯 What: The load_payload function in bin/idstack-manifest-merge lacked validation to ensure the parsed JSON payload was a dictionary. This could lead to runtime errors if users provided a list or string payload.
📊 Coverage: Added two new unit tests in test/test-manifest-merge.sh to test rejecting payloads with a list root and a string root. Both test that bin/idstack-manifest-merge exits with code 1.
✨ Result: Improved test coverage and reliability by explicitly validating the payload type in bin/idstack-manifest-merge and testing it, preventing unexpected downstream behaviors.

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PR created automatically by Jules for task 3643888697011172478 started by @savvides

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[gemini-code-assist]

Code Review

This pull request updates the load_payload function in bin/idstack-manifest-merge to ensure that the parsed JSON payload root is a dictionary (JSON object), exiting with an error code if it is not. It also adds corresponding integration tests in test/test-manifest-merge.sh to verify this behavior for list and string roots. There are no review comments, and I have no feedback to provide.

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