feat(certificates): cert Customer Contact is multi-contact, auto-filled, sends to all

fp.certificate.contact_partner_id (single 'Customer Contact') becomes
contact_partner_ids (Many2many) — same shape as the partner's Default CoC
Contacts, as requested.

- Auto-populate: at job creation (fp.job cert resolution) + lazy-fill at issue,
  contact_partner_ids = the customer's x_fc_default_coc_contact_ids (ALL).
- Send: action_send_to_customer pre-fills the composer with exactly the cert's
  contact_partner_ids, so the CoC goes to all the defined clients (fallback:
  company).
- Primary: the FIRST contact prints on the CoC + is gated for email; report
  uses contact_partner_ids[:1].
- Gate: requires >=1 Customer Contact + the primary has an email.
- View: many2many_tags.
- Migration 19.0.10.3.0: copies each cert's old single contact into the new M2m,
  drops the orphaned column.

Deployed + verified on entech: migration copied 16 certs, old column dropped,
field is M2m, send pre-fills the cert contacts, CoC report renders. entech-only
part_line_ids preserved.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

fusion_plating/fusion_plating_jobs/tests/test_fp_job_milestone_cascade.py

@@ -673,7 +673,7 @@ class TestCertCreationAndGates(TransactionCase):
         cert = self.env['fp.certificate'].search([
             ('x_fc_job_id', '=', job.id),
         ])
-        self.assertEqual(cert.contact_partner_id, self.contact)
+        self.assertEqual(cert.contact_partner_ids, self.contact)
 
     def test_create_cert_computes_nc_quantity(self):
         job = self._make_job(

fusion_plating/fusion_plating_jobs/tests/test_recipe_cert_suppression.py

@@ -130,7 +130,7 @@ class TestRecipeCertSuppression(TransactionCase):
             'certificate_type': 'nadcap_cert',
             'state': 'draft',
             'partner_id': self.partner.id,
-            'contact_partner_id': self.partner.id,
+            'contact_partner_ids': [(6, 0, [self.partner.id])],
             'spec_reference': 'AMS 2404',
             'process_description': 'TEST PROCESS',
             'certified_by_id': self.env.user.id,