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_certificates/tests/test_action_issue_gates.py

@@ -50,7 +50,7 @@ class TestActionIssueGates(TransactionCase):
             'spec_reference':      'AMS 2404',
             'process_description': 'ELECTROLESS NICKEL PER AMS 2404',
             'certified_by_id':     self.signer.id,
-            'contact_partner_id':  self.contact_with_email.id,
+            'contact_partner_ids': [(6, 0, [self.contact_with_email.id])],
         }
         vals.update(kw)
         return self.env['fp.certificate'].create(vals)
@@ -85,13 +85,13 @@ class TestActionIssueGates(TransactionCase):
     # ---- new gate: contact_partner_id ----
 
     def test_blocks_on_missing_contact(self):
-        cert = self._make_cert(contact_partner_id=False)
+        cert = self._make_cert(contact_partner_ids=False)
         with self.assertRaises(UserError) as exc:
             cert.action_issue()
         self.assertIn('Customer Contact', str(exc.exception))
 
     def test_blocks_on_contact_without_email(self):
-        cert = self._make_cert(contact_partner_id=self.contact_no_email.id)
+        cert = self._make_cert(contact_partner_ids=[(6, 0, [self.contact_no_email.id])])
         with self.assertRaises(UserError) as exc:
             cert.action_issue()
         self.assertIn('no email', str(exc.exception))
@@ -113,7 +113,7 @@ class TestActionIssueGates(TransactionCase):
             spec_reference=False,
             process_description=False,
             certified_by_id=False,
-            contact_partner_id=False,
+            contact_partner_ids=False,
         )
         with self.assertRaises(UserError) as exc:
             cert.action_issue()