[FIX] borders: preserve side borders when combining External and All

When applying an External border on a range and then applying All borders on
an inner/overlapping range, the side borders (left/right) of the inner cells
were lost after re-opening the spreadsheet.

The exported border data was correct, but the BordersPlugin recomputation
logic in addBorder incorrectly cleared adjacent borders on import/update. The
adjacency check (adjacent(existingBorder.zone, zone)) reported the side of the
existing zone, but we were deciding whether to clear it based on the same side
key in the new border, instead of the opposite side (shared edge) on the new
zone. As a result, importing the combination of:

C2:C4 with All borders
B2:B4 with left/top/bottom
D2:D4 with right/top/bottom

ended up clearing the left and right borders of C2:C4.

This commit adjusts the adjacent clearing logic to:

Map the adjacent side of the existing zone to the opposite side on the new
zone.

Only clear the existing side if the corresponding opposite side is actually
being written on the new border.

Task: 5270171

Author: dhrp-odoo

src/plugins/core/borders.ts

@@ -343,10 +343,10 @@ export class BordersPlugin extends CorePlugin<BordersPluginState> implements Bor
     const borders: ZoneBorder[] = [];
     const plannedBorder = newBorder ? { zone, style: newBorder } : undefined;
     const sideToClear = {
-      left: force || !!newBorder?.left,
-      right: force || !!newBorder?.right,
-      top: force || !!newBorder?.top,
-      bottom: force || !!newBorder?.bottom,
+      left: force || !!newBorder?.right,
+      right: force || !!newBorder?.left,
+      top: force || !!newBorder?.bottom,
+      bottom: force || !!newBorder?.top,
     };
     let editingZone: Zone[] = [zone];
     for (const existingBorder of this.borders[sheetId] ?? []) {

tests/borders/border_plugin.test.ts

@@ -147,6 +147,36 @@ describe("borders", () => {
     });
   });
 
+  test("Preserves side borders when combining external and all via command", () => {
+    const model = new Model();
+    const defaultBorder = DEFAULT_BORDER_DESC;
+
+    setBordersOnTarget(model, ["C2:C4"], {
+      top: defaultBorder,
+      bottom: defaultBorder,
+      left: defaultBorder,
+      right: defaultBorder,
+    });
+    setZoneBorders(model, { position: "hv" }, ["C2:C4"]);
+    setBordersOnTarget(model, ["B2:B4"], {
+      top: defaultBorder,
+      bottom: defaultBorder,
+      left: defaultBorder,
+    });
+    setBordersOnTarget(model, ["D2:D4"], {
+      top: defaultBorder,
+      bottom: defaultBorder,
+      right: defaultBorder,
+    });
+
+    expect(getBorder(model, "C2")).toEqual({
+      top: defaultBorder,
+      bottom: defaultBorder,
+      left: defaultBorder,
+      right: defaultBorder,
+    });
+  });
+
   test("can set all borders in a zone", () => {
     const model = new Model();