Add try-catch for JSON parsing in proxy convention

[todo-mdx]

If upstream returns malformed JSON with application/json content-type, the proxy throws an unhandled error.

Location: src/conventions/proxy.ts lines 68-76

TDD Steps:

• RED: Write test with mock upstream returning invalid JSON with json content-type
• GREEN: Wrap response.json() in try-catch, return 502 with error details
• REFACTOR: Consider similar handling in other conventions