Add and validate automated SplitContainer continuous-repaint coverage

[KlausLoeffelmann]

Objective

Create automated coverage for continuous SplitContainer panel repainting in VisualStylesMode.Net11 and later, then run the coverage locally and in CI.

Test requirements

• Add focused tests that fail against the regression where practical and pass with the fix.
• Cover both panel orientations and representative splitter movement.
• Cover mouse-driven behavior and keyboard movement where test infrastructure permits.
• Verify mode gating: Net11 and later receive the new behavior while Classic mode remains correct.
• Cover handle-created and non-handle-created states where relevant.
• Verify final splitter position, layout bounds, invalidation/repaint effects, and event behavior without asserting implementation details unnecessarily.
• Include regression coverage for re-entrant layout/paint behavior and excessive invalidation if the chosen implementation introduces such risk.
• Follow existing WinForms test conventions and avoid timing-fragile assertions.

Execution

• Run the smallest relevant test set locally during development.
• Run the complete affected test project locally before completion.
• Ensure the tests run in the normal CI build and investigate any environment-specific failures.
• Record the exact local test command and link the successful CI run.

Completion criteria

• Regression coverage is present and meaningful.
• Mode-specific and Classic-mode behavior are covered.
• Relevant tests pass locally.
• The affected test project passes locally.
• CI completes successfully with the new tests enabled.
• No tests are skipped or disabled merely to meet the schedule.