fix: [sc-87908] JSON Marshal Errors Silently Swallowed in result.go

[mlataza]

Summary

• errorResultBytes and resultBytes in internal/interpreter/result.go were discarding json.MarshalIndent errors with _, returning nil bytes on failure with no log entry and no signal to the caller
• Both functions now accept an hclog.Logger, check the marshal error, log it via logger.Error, and return a hardcoded valid JSON fallback so neither function can ever return nil or empty bytes
• All call sites in base_executor.go and message.go updated to pass the already-available logger; tests updated accordingly with new assertions that returned bytes are always non-nil and valid JSON

Test plan

• Run go test ./internal/interpreter/... — all tests pass
• Run go vet ./internal/interpreter/... — no new warnings
• Confirm TestErrorResultBytesNeverNil and TestResultBytesNeverNil assert non-empty, valid JSON output from both functions
• Confirm existing TestErrorResultBytes and TestResultBytes still validate correct field values