[CT-86] error.rs has no test that internal error variants never leak stack traces or file paths

[mftee]

Overview

contract/src/error.rs (80 lines) defines the crate's error types and their API-facing representation, but there's no evident test asserting an internal/unexpected error variant's HTTP response body excludes things like file paths, dependency error debug output, or stack traces — details useful to an attacker probing the service.

Tasks

• Add a test for each error variant asserting the serialized API response contains only the intended user-facing message
• Specifically test that a wrapped internal/std error's Debug output never leaks into the response body

Acceptance Criteria

• Every error variant's API response is asserted to exclude internal debug detail, file paths, and stack traces

Notes for Contributors

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