#170: Fix ETL certificate syntax for Exasol 7.1

[tkilias]

Fixes #170

Summary

• Version-gate IGNORE CERTIFICATE in exa.readData() and exa.writeData().
• Fail closed when the Exasol DB version cannot be determined.
• Add a regression test for the certificate-clause helper.

Root cause

The previous change always appended IGNORE CERTIFICATE to IMPORT/EXPORT SQL. That is rejected by older Exasol 7.1 clusters, while newer clusters require the clause for encrypted ETL traffic.

Validation

• Rscript parse checks for the edited R files
• Direct helper checks for pre-2025.1, 2025.1.0+, and unknown version strings