fix(QgsCurve): properly handle area cache when reversing geometry

[lbartoletti]

Description

@agiudiceandrea I was wrong in my previous PR about the sumUpArea method. While unconventional in my opinion, this method works correctly, particularly with compound curves where the "classical" approach would fail.

The issue appears to be cache-related.
When reversing a line or circular string geometry, ensure the cached area value is also reversed by negating it. This ensures correct area orientation calculations, particularly important for:

• Maintaining correct winding order information
• Ensuring consistent orientation() results
• Preserving area sign conventions (positive for CCW, negative for CW)

Added unit tests verifying the area and orientation behavior for both clockwise and counter-clockwise line strings, and their reversed versions.

Follow-up #59156
Fixes #58333

[github-actions]

🪟 Windows builds

Download Windows builds of this PR for testing.
Debug symbols for this build are available here.
(Built from commit 61b7291)