fix: answer 404 for a conversation, call or Dot that does not exist

[charan-rathore]

Problem

On the workspace routes, asking for a conversation, call or Dot that does not exist answers 503 with "The service request failed. Check the server configuration and try again." Examples: GET /api/conversations/<unknown>/capture and GET /api/voice/calls/<unknown>. A missing resource is a caller error, so the status and message send the user to check the server configuration for something that is not wrong with the server.

Change

• The onError handler in workspaceRoutes now answers 404 with the error's own message for "Dot not found", "Call not found" and "Conversation does not belong ..." instead of 503.
• Real Intelligence and setup failures still answer 503, unchanged.
• Regression test in tests/page-routes.test.ts for the conversation capture and call routes.

Testing

typecheck, eslint, prettier check and the full vitest suite (234 tests) pass. The new test fails without the change to workspace-routes.ts and passes with it.