fix(configurator): programmatic SO-line create fallback for x_fc_internal_description

When Sub 2 Task 26 flipped x_fc_internal_description to required=True,
any programmatic sale.order.line creation that doesn't set the field
fails at the Postgres NOT NULL constraint. Callers include:
 - sale_mrp stock-move line creation (doesn't set name either)
 - demo seeders
 - external integrations
 - test scripts

The UI-side onchange populates the field when the user picks a
description template; this hook mirrors that for programmatic callers.
Fallback chain: explicit vals['x_fc_internal_description'] → vals['name']
→ product_id.display_name → '—'. Matches the migration's backfill rule.

Also adds Sub 2 end-to-end smoke test (6 cases, all green):
 1. Required-field rejection on part creation
 2. Required-field rejection on template creation
 3. Template picker populates both SO-line descriptions
 4. Cert resolver: part-level override wins over partner
 5. display_name renders part_number + revision + name
 6. certificate_requirement defaults to 'inherit'

QC Phase 1-3 regression suite remains green after the fix.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

fusion_plating/fusion_plating_configurator/models/sale_order_line.py

@@ -67,6 +67,36 @@ class SaleOrderLine(models.Model):
              'preserved for audit. Useful when a part is cancelled mid-order.',
     )
 
+    @api.model_create_multi
+    def create(self, vals_list):
+        """Default `x_fc_internal_description` from `name` when a caller
+        creates a line programmatically without supplying the internal
+        description.
+
+        Sub 2 made `x_fc_internal_description` required. The UI-side
+        onchange fills it when the user picks a description template,
+        but programmatic creators (sale_mrp bridge, migration scripts,
+        external integrations, demo seeders) may not know about this
+        field. Instead of forcing every call site to update, fall back
+        to `name` — same rule the upgrade migration used when it
+        back-filled historical lines.
+        """
+        Product = self.env['product.product']
+        for vals in vals_list:
+            if vals.get('x_fc_internal_description'):
+                continue
+            # Try the explicit `name` first. If the caller didn't pass
+            # one (sale_mrp + some Odoo internals don't — they let the
+            # name compute from product_id later), fall back to the
+            # product's display_name so we have SOMETHING non-empty.
+            fallback = vals.get('name')
+            if not fallback and vals.get('product_id'):
+                prod = Product.browse(vals['product_id']).exists()
+                if prod:
+                    fallback = prod.display_name or prod.name
+            vals['x_fc_internal_description'] = fallback or '—'
+        return super().create(vals_list)
+
     @api.onchange('x_fc_description_template_id')
     def _onchange_description_template(self):
         """When estimator picks a template, auto-fill both descriptions.