Add configurable Oracle Thick mode with NNE CI

[Maxteabag]

Summary

• add Thin/Thick selection to Oracle connection settings
• show the optional Oracle Client library directory only for Thick mode
• initialize Oracle Client before connecting and report actionable DPI errors
• retain Oracle Legacy Thick behavior through the shared initializer
• reject a Thin selection when the process has already initialized Thick mode
• extend the existing Oracle CI service to require AES256 NNE and SHA512 integrity
• download and checksum-pin Oracle Instant Client Basic Light 23.26.3
• prove Thin mode fails with DPY-3001 and Thick mode connects/query succeeds
• pin the Oracle Free service image by digest

Fixes #300.

Validation

• local NNE-required Oracle Free container reproduced the reporter’s exact DPY-3001 error
• real Instant Client Thick connection succeeded and returned SELECT 1 FROM dual
• NNE integration + Oracle adapter unit suite: 38 passed
• Ruff and structured autoreview: clean

This PR is opened to exercise the complete GitHub Actions fixture and is not merged yet.