[PM-28555] Add idempotent sproc to create My Items collections (#6801)

* Add sproc to create multiple default collections. 
  SqlBulkCopy implementation is overkill for most cases.
  This provides a lighter weight sproc implementation for smaller
  data sets.
* DRY up collection arrangement
* DRY up tests because bulk and non-bulk share same behavior
* use EF native AddRange instead of bulk insert, because
  we expect smaller data sizes on self-host

util/Migrator/DbScripts/2026-01-13_00_Collection_CreateDefaultCollections.sql

@@ -0,0 +1,70 @@
+-- Creates default user collections for organization users
+-- Filters out existing default collections at database level
+CREATE OR ALTER PROCEDURE [dbo].[Collection_CreateDefaultCollections]
+    @OrganizationId UNIQUEIDENTIFIER,
+    @DefaultCollectionName VARCHAR(MAX),
+    @OrganizationUserCollectionIds AS [dbo].[TwoGuidIdArray] READONLY -- OrganizationUserId, CollectionId
+AS
+BEGIN
+    SET NOCOUNT ON
+
+    DECLARE @Now DATETIME2(7) = GETUTCDATE()
+
+    -- Filter to only users who don't have default collections
+    SELECT ids.Id1, ids.Id2
+    INTO #FilteredIds
+    FROM @OrganizationUserCollectionIds ids
+    WHERE NOT EXISTS (
+        SELECT 1
+        FROM [dbo].[CollectionUser] cu
+        INNER JOIN [dbo].[Collection] c ON c.Id = cu.CollectionId
+        WHERE c.OrganizationId = @OrganizationId
+          AND c.[Type] = 1 -- CollectionType.DefaultUserCollection
+          AND cu.OrganizationUserId = ids.Id1
+    );
+
+    -- Insert collections only for users who don't have default collections yet
+    INSERT INTO [dbo].[Collection]
+    (
+        [Id],
+        [OrganizationId],
+        [Name],
+        [CreationDate],
+        [RevisionDate],
+        [Type],
+        [ExternalId],
+        [DefaultUserCollectionEmail]
+    )
+    SELECT
+        ids.Id2, -- CollectionId
+        @OrganizationId,
+        @DefaultCollectionName,
+        @Now,
+        @Now,
+        1, -- CollectionType.DefaultUserCollection
+        NULL,
+        NULL
+    FROM
+        #FilteredIds ids;
+
+    -- Insert collection user mappings
+    INSERT INTO [dbo].[CollectionUser]
+    (
+        [CollectionId],
+        [OrganizationUserId],
+        [ReadOnly],
+        [HidePasswords],
+        [Manage]
+    )
+    SELECT
+        ids.Id2, -- CollectionId
+        ids.Id1, -- OrganizationUserId
+        0, -- ReadOnly = false
+        0, -- HidePasswords = false
+        1  -- Manage = true
+    FROM
+        #FilteredIds ids;
+
+    DROP TABLE #FilteredIds;
+END
+GO