Steps to reproduce:
Bug 1 (The Crash):
1. Open Documents app, select a PDF, and click Action > Sign.
2. In the Sign app, delete the newly created Sign Template.
3. Return to the Documents app.
4. A traceback occurs (`KeyError: <document_id>`) in `web_read`.

Bug 2 (The Broken Lineage):
1. Create two separate Sign Templates from the exact same Document.
2. Send a signature request from the second template.
3. The `reference_doc` on the signature request fails to link back to the original Document.

Current behavior:
When creating a sign template from a document, `documents_sign` intentionally unlinks the original `ir.attachment` (`res_model = False`) to pass custody to `sign.document`. If the template is deleted, the attachment is orphaned, permanently corrupting the original `documents.document` and crashing the UI. 
Furthermore, the lineage tracking (`reference_doc`) relies strictly on a 1:1 shared `attachment_id`. If a user creates multiple templates from one document, the system is forced to make a copy for the second template, natively breaking the lineage tracking because the IDs no longer match.

Expected behavior:
Documents should not be corrupted when generating or deleting sign templates. Furthermore, lineage tracking (`reference_doc`) should successfully link back to the original document regardless of how many templates have been generated from it.

Fix:
1. Replaced the `res_model = False` custody-handoff hack in `documents_sign` with a safe `.copy({'original_id': attachment.id})`. This sandboxes the Sign app's files, completely preventing the deletion crash and the multi-template conflicts.
2. Updated the `reference_doc` computation in `sign.request` to dynamically search for both the current `attachment_id` AND its `original_id` (utilizing a minimal-diff recordset union `|`). This perfectly preserves the lineage tracking for all templates without requiring database schema changes.

Task: 5432116

Forward-Port-Of: odoo/enterprise#114221
Forward-Port-Of: odoo/enterprise#113167