Fix: reduce MSSQL connections per collect; add unit tests

[Rom1-B]

Checklist before requesting a review

• I have performed a self-review of my code.
• I have added tests (when available) that prove my fix is effective or that my feature works.
• I have updated the CHANGELOG with a short functional description of the fix or new feature.
• This change requires a documentation update.

Description

• Need Feat collection #174

Each collect run was opening up to N*8 MSSQL connections (one per get*() method per device).
A single PluginSccmSccmdb is now created once per config in executeCollect() and passed to all per-device data fetching methods, reducing connections to 2 per config regardless of device count.
The set*() methods in PluginSccmSccmxml accept an optional ?PluginSccmSccm parameter for dependency injection, enabling a PHPUnit test suite covering query building, XML generation, and data transformation logic (47 tests, 209 assertions).

[tolemac]

@Rom1-B Tell me when I can test it in my environment.