Honor template content reuse during row recycling

[wieslawsoltes]

Summary

Honor DataGridTemplateColumn.ReuseCellContent when a recycled row is assigned to another ordinary data item, while retaining forced regeneration for template changes and edit-mode transitions.

Fixes #290.

Root cause

DataGridRow.OnPropertyChanged refreshed template cells after a recycled row received a new DataContext by calling RefreshCellContent with nameof(DataGridTemplateColumn.CellTemplate).

That refresh reason is also used for a real template-property change. DataGridTemplateColumn therefore set _forceGenerateCellFromTemplate, discarded the existing control, and rebuilt the template. In practice, every real-to-real row recycle bypassed ReuseCellContent, so the optimization worked in direct GenerateElement tests but not through the actual row lifecycle.

Changes

• Separate data-context refresh from template-property refresh.
• Add an internal data-context refresh path that allows the existing cell content to participate in normal reuse/recycling decisions.
• Keep explicit template changes on a forced-build path so replacing CellTemplate or NewRowCellTemplate cannot retain visuals from the old template.
• Restrict the one-shot force flag to its original edit-mode responsibility: replacing an editing control with display content when a cell exits edit mode.
• Preserve the existing placeholder-transition handling, which clears incompatible visuals before moving between a real item and the new-item placeholder.
• Add an Avalonia Headless lifecycle regression test that recycles a row between two ordinary items and verifies both control identity and template build count.

User and performance impact

Applications opting into ReuseCellContent now receive the intended behavior during virtualization: ordinary row recycling retains the existing control and lets inherited DataContext update it. This avoids repeated template construction in scroll/update paths without changing the default behavior when reuse is disabled.

Validation

• Template reuse, editing, and container lifecycle suites: 24 passed, 0 failed.
• Full Avalonia.Controls.DataGrid.UnitTests project: 2,251 passed, 0 failed.
• git diff --check: clean.

[wieslawsoltes]

Self-review completed. I found and fixed an edge case where content created from NewRowCellTemplate could be reused after the placeholder became a regular data row. Commit 86349f7 uses the row recycling placeholder state to force the correct display template and adds regression coverage. Focused tests passed 4/4, the full unit suite passed 2,252/2,252, all six CI checks are successful, and git diff --check is clean.