Upsert drop: clean up child-side metadata for two-way relationships

On a first-run migration createField writes TWO metadata documents for a
two-way relationship attribute: parent-side at
{dbSeq}_{tableSeq}_{key} (line 763) and child-side at
{dbSeq}_{relatedTableSeq}_{twoWayKey} (line 819). Migration operates
directly on the database via $dbForProject — it doesn't rely on
Appwrite's attribute-event workers to derive the child side — so both
documents are physically written by migration itself.

The Upsert drop block was only removing the parent-side document +
column. On the recreate pass that followed, line 819 re-wrote the
child-side document and immediately hit DuplicateException because the
stale child doc from the previous migration run was still sitting there.
The inner catch rolled back the parent and the whole attribute aborted
with "Attribute already exists" — breaking Upsert re-migration for every
two-way relationship whose source spec had changed.

Fix: in the Upsert drop branch, when the resource is a two-way
relationship, also delete the child-side metadata document and the
child-side physical column on the related table, then purge caches.
Both deletes are wrapped in try/catch to tolerate the case where a prior
interrupted run (or utopia-php/database's relationship handling) already
cleaned one side — the goal is to guarantee a clean slate for the
downstream recreate, not to require both sides to still exist.

Flagged by greptile P1. Reproducing scenario:
  - OnDuplicate::Upsert
  - Destination already contains the attribute
  - Source's two-way relationship spec was modified between runs
    (source.updatedAt > dest.updatedAt)

Author: premtsd-code

src/Migration/Destinations/Appwrite.php

@@ -732,6 +732,37 @@ protected function createField(Column|Attribute $resource): bool
                 $this->dbForProject->deleteDocument('attributes', $attributeMetaId);
                 $this->dbForProject->purgeCachedDocument('database_' . $database->getSequence(), $table->getId());
                 $dbForDatabases->purgeCachedCollection('database_' . $database->getSequence() . '_collection_' . $table->getSequence());
+
+                // Two-way relationships: the first-run createField wrote TWO
+                // metadata docs (parent at line 763, child at line 819). The
+                // drop above only removed the parent; without cleaning up the
+                // child, the recreate path hits DuplicateException at line 819
+                // and the whole attribute re-migration fails.
+                $resourceOptions = $resource->getOptions();
+                if ($type === UtopiaDatabase::VAR_RELATIONSHIP && !empty($resourceOptions['twoWay'])) {
+                    // $relatedTable was populated at the top of createField
+                    // inside the same VAR_RELATIONSHIP branch.
+                    $childTwoWayKey = $resourceOptions['twoWayKey'] ?? '';
+                    if ($childTwoWayKey !== '') {
+                        $childMetaId = $database->getSequence() . '_' . $relatedTable->getSequence() . '_' . $childTwoWayKey;
+                        try {
+                            $this->dbForProject->deleteDocument('attributes', $childMetaId);
+                        } catch (\Throwable) {
+                            // Child metadata already gone — interrupted prior run, nothing to do.
+                        }
+                        try {
+                            $dbForDatabases->deleteAttribute(
+                                'database_' . $database->getSequence() . '_collection_' . $relatedTable->getSequence(),
+                                $childTwoWayKey
+                            );
+                        } catch (\Throwable) {
+                            // Child column already gone — the parent's deleteAttribute may
+                            // have cascaded via utopia-php/database's relationship handling.
+                        }
+                        $this->dbForProject->purgeCachedDocument('database_' . $database->getSequence(), $relatedTable->getId());
+                        $dbForDatabases->purgeCachedCollection('database_' . $database->getSequence() . '_collection_' . $relatedTable->getSequence());
+                    }
+                }
             }
         }