Add missing indexes to call stack columns in EF Core migrations

[Copilot]

The V3_7 migrations added call stack tracking columns (SchedulingActivityExecutionId, SchedulingActivityId, SchedulingWorkflowInstanceId) but omitted the indexes required by the plan document. These columns are used in WHERE clauses and recursive chain traversal, causing table scans on unindexed columns.

Changes

• Added indexes on all three call stack columns across all database providers (SQLite, PostgreSQL, Oracle, SQL Server, MySQL)
• Fixed column types for indexability:
  • SQL Server: nvarchar(max) → nvarchar(450) (matches other ID columns)
  • MySQL: longtext → varchar(255) (matches other ID columns)
  • Oracle/PostgreSQL/SQLite: No change needed (already indexable)
• Added corresponding DropIndex calls in migration Down methods

Query Context

// ActivityExecutionRecordFilter.Apply() uses these in WHERE clauses
if (filter.SchedulingActivityExecutionId != null) 
    queryable = queryable.Where(x => x.SchedulingActivityExecutionId == filter.SchedulingActivityExecutionId);

// GetExecutionChainAsync() traverses the chain recursively
while (currentId != null && visited.Add(currentId))
{
    record = await store.FindAsync(new() { Id = currentId }, cancellationToken);
    currentId = record.SchedulingActivityExecutionId;  // Chain traversal
}

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