fix(task_sync): defend against silent sync_id integrity violations

The cross-instance sync silently drops tasks when x_fc_tech_sync_id is
missing on the technician, and silently collapses duplicates via dict
comprehension. Both make sync break in ways that are invisible until
someone notices a missing task on the other instance.

- _get_remote_tech_map / _get_local_syncid_to_uid: warn on duplicates
- _push_tasks_to_remote: info-log when a task is skipped because the
  tech has no sync_id or no remote counterpart
- res.users onchange: warn in the form when entering a sync_id that
  is already used by another active field staff

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

fusion_tasks/models/res_users.py

@@ -2,7 +2,7 @@
 # Copyright 2024-2026 Nexa Systems Inc.
 # License OPL-1 (Odoo Proprietary License v1.0)
 
-from odoo import models, fields
+from odoo import models, fields, api
 
 
 class ResUsers(models.Model):
@@ -24,3 +24,26 @@ class ResUsers(models.Model):
              'Must be the same value on all instances for the same person.',
         copy=False,
     )
+
+    @api.onchange('x_fc_tech_sync_id')
+    def _onchange_x_fc_tech_sync_id_dup_warning(self):
+        if not self.x_fc_tech_sync_id:
+            return
+        dup = self.env['res.users'].sudo().search([
+            ('id', '!=', self._origin.id or self.id),
+            ('x_fc_tech_sync_id', '=', self.x_fc_tech_sync_id),
+            ('x_fc_is_field_staff', '=', True),
+            ('active', '=', True),
+        ], limit=1)
+        if dup:
+            return {
+                'warning': {
+                    'title': "Duplicate Tech Sync ID",
+                    'message': (
+                        f"Tech Sync ID {self.x_fc_tech_sync_id!r} is already used "
+                        f"by {dup.login} ({dup.partner_id.name}). Cross-instance "
+                        f"task sync only routes to ONE user per sync ID — "
+                        f"pick a unique value or only one tech's tasks will sync."
+                    ),
+                }
+            }