user-service returns 400 instead of 404 for missing profiles

[kenahrens]

Problem\nThe user-service profile endpoint maps User not found to 400 Bad Request instead of 404 Not Found.\n\n## Expected\nMissing user profiles should return 404 so client behavior matches a not-found resource.\n\n## Acceptance criteria\n- GET /api/users/profile returns 404 when the authenticated user does not exist\n- controller test covers the not-found case\n- proxymock replay continues to pass after the change

[spd-agent-factory-bot]

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