[iyanukid2008-pixel]

Summary

• Add a regression test using a real xdr.DiagnosticEvent to verify the diagnostic contract hash is converted to the expected C-address in the extracted failure string.
• Log unexpected diagnostic XDR parsing failures at debug level instead of silently swallowing them.
• The explicit Buffer conversion that satisfies StrKey.encodeContract is present in the current base; this change adds regression coverage for its runtime behavior.

Closes #499

Validation

• npx vitest run lib/contract-confirm.test.ts passed (2 tests).
• npm run typecheck passed.
• npm run lint passed with 7 existing warnings.
• NEXT_PUBLIC_MOCK_API=true npm run build passed.
• npm test reported 45 failing tests across 10 files and 911 passing tests; failures are outside the new test file and cover existing app/configuration, fixture, and RPC-mock tests.