fix(traverse_lineage): return graceful message on 404 instead of raw error

[mothership-ai]

Session owner: @abhinavmathur-atlan

Summary

• AICHAT-818: traverse_lineage was propagating raw NotFoundError (404) messages to users when an asset has no lineage or the GUID doesn't exist in the lineage graph
• Added a specific NotFoundError catch before the generic Exception handler that returns a user-friendly message with error: None, matching the existing "no results" pattern
• 16 errors observed in 7 days in Braintrust — all will now return a clean response instead

Changes

• modelcontextprotocol/tools/lineage.py: Import NotFoundError from pyatlan.errors, add specific 404 handling that returns {"assets": [], "error": None, "message": "No lineage found for this asset..."}

Test plan

• Verify with an asset GUID that has no lineage — should return graceful message instead of raw 404 error
• Verify with a non-existent GUID — should return graceful message
• Verify with an asset that has lineage — should continue to return results normally
• Verify other exceptions (e.g., auth errors) still propagate via the generic handler

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