Refresh cell validation after programmatic changes

[wieslawsoltes]

Summary

Refresh realized DataGrid cell validation immediately when an item raises INotifyDataErrorInfo.ErrorsChanged after a programmatic property change.

Fixes #287.

Problem

The collection-level ErrorsChanged handler recomputed grid validity but did not restore validation on realized cells. Visible severity, pseudo-classes, and DataValidationErrors could therefore remain stale until another grid interaction.

Changes

• Refresh realized rows for the item that raised ErrorsChanged.
• Reuse the row-validation restoration path for consistent bound-column lookup, severity, pseudo-classes, and displayed errors.
• Refresh non-edited cells on the active editing row while leaving the editor-owned column untouched.
• Track INDEI-owned displayed errors separately from independent binding/setter errors.
• Expose source-write validation provenance to public custom ICellEditBinding implementations with a compatibility-preserving default.
• Derive edit acceptance and cell/row/grid severity from the final merged binding, preserved, and INDEI error set.
• Release per-cell ownership at the common cell-removal boundary used by dynamic column removal and cell rebuilds.
• Add Avalonia Headless and Release leak regressions for programmatic transitions, mixed validation sources, edit ownership, custom bindings, merged severity, and removed-cell collection.

User impact

ViewModel and service-driven changes now surface or clear visible validation immediately. Clearing a model error no longer removes an independent warning on the same cell, even when it has the same message and severity, and this behavior also applies to custom edit bindings that declare source-write provenance. Edit validity consistently reflects the most severe displayed error. Dynamic column/cell rebuilds do not retain detached validated cells.

Review hardening

Follow-up fixes:

• preserve unrelated and same-cell binding validation during INDEI refresh;
• refresh sibling cells on an editing row without disturbing the editor;
• transfer ownership when edit mode hides or replaces a cell error;
• claim INDEI warnings introduced by an edit;
• partition mixed edit aggregates by provenance;
• retain setter/binding errors that have the same message and severity as an INDEI error by using the binding engine source-write result;
• expose the provenance marker through the public edit-binding contract for third-party columns;
• cover default-mode bindings after DataGrid normalizes them to two-way editing;
• merge binding, preserved, and INDEI severities before deciding edit acceptance and visual validity;
• evict ownership when a cell leaves its row, preventing the grid dictionary from retaining removed cells.

Validation

• DataGridValidationTests: 28 passed, 0 failed.
• Full Avalonia.Controls.DataGrid.UnitTests: 2,261 passed, 0 failed.
• Release leak suite: 35 passed, 0 failed.
• git diff --check: clean.
• All review threads resolved.

[wieslawsoltes]

Self-review completed. I reviewed the validation refresh path and its regression coverage; no additional actionable findings were found. All six CI checks are successful, the branch is synchronized, and git diff --check is clean. This PR is ready for review.

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