feat: migrate "Regression" user stories from Robot to pytest

[tarilabs]

Description

• Add structured error message tests for malformed model data (migrated from Robot)
• Test API returns proper error messages for malformed RegisteredModel and ModelVersion requests
  • for both code and message
• Clean ups/linting

follow-up from #1331

How Has This Been Tested?

local make test-e2e

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• All the commits have been signed-off (To pass the DCO check)

• The commits have meaningful messages
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If you have UI changes

• The developer has added tests or explained why testing cannot be added.
• Included any necessary screenshots or gifs if it was a UI change.
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[fege]

/lgtm

[tarilabs]

bringing this is

/approve

[google-oss-prow]

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