Create Harbor Saved Query Draft

Creates a stable Harbor saved query with its initial mutable draft. Saved queries are supported only for Polytomic-managed Harbors whose backing Connection type is `polytomic_harbor`. Customer-managed Harbor backings return an unsupported-backing error. The saved query receives a stable ID, but it remains absent from published saved-query reads until an administrator publishes its initial draft. Use `{{parameter_name}}` references in `sql_template`. Each reference must have one scalar declaration. Validation values and defaults must match the declared `string`, `number`, `boolean`, `date`, `timestamp`, or `uuid` type. Unknown JSON fields, including fields inside parameter declarations, return `400 Bad Request` before the draft is saved. Use `default_value`, not `default`, for parameter defaults. When you omit a parameter during execution, Polytomic uses its published `default_value`, never its draft validation value. An omitted optional parameter without a default binds SQL `NULL`. A required parameter allows omission when a default exists, but rejects explicit `null` even with a default. For a report window, use a required parameter with a default so omission selects a useful window and explicit `null` is rejected: ```json {"name": "days", "type": "number", "required": true, "default_value": 7} ```