[18.0] [FIX] subscription_oca: add migration script for subscription_template_id field on product.template model

[rrebollo]

Context

We discovered this issue during a customer migration from v14 to v18: the subscription_template_id field value for product templates is lost after upgrading to v18.

Root Cause

#1060 made subscription_template_id a company-dependent field on product.template, but this change was never forward-ported beyond its target version (v14).

In Odoo 18.0, company-dependent fields are stored in a JSONB column instead of ir_property records. Since the forward-port never happened, the automatic migration from ir_property to JSONB did not occur, leaving the field empty after upgrade.

Solution

This pull request adds a pre-migration script for Odoo 18.0 that backfills product_template.subscription_template_id from legacy ir_property records, ensuring existing data is preserved during the upgrade.

Scope

This fix applies in a way that only instances affected by the company-dependent implementation of the subscription_template_id field will be targeted. Nothing changes or backfires if you're not affected.

Migration and Data Integrity

• Added pre-migration.py to backfill product_template.subscription_template_id from legacy ir_property records
• Documented the root cause and provided context about the storage change for company-dependent fields

[luisDIXMIT]

Code review, LGTM!

[cvinh]

LGTM code review no test

[OCA-git-bot]

This PR has the approved label and has been created more than 5 days ago. It should therefore be ready to merge by a maintainer (or a PSC member if the concerned addon has no declared maintainer). 🤖

[github-actions]

There hasn't been any activity on this pull request in the past 4 months, so it has been marked as stale and it will be closed automatically if no further activity occurs in the next 30 days.
If you want this PR to never become stale, please ask a PSC member to apply the "no stale" label.

[rrebollo]

@pedrobaeza any chances to get this merged?

[pedrobaeza]

Maintainers of this modules should trigger it, as I don't use (and you know that I don't want this duality contract/subscription_oca.

[rrebollo]

@rousseldenis @sbejaoui would you merge this one?