fix(m3u): improve RANGE_EXHAUSTED error reporting for provider mode

- Introduced a new helper function to generate user-facing error messages for RANGE_EXHAUSTED failures, ensuring the fallback range is correctly displayed.
- Updated tests to verify that the error message reflects the fallback range instead of the hidden auto_sync_channel_start, enhancing clarity for users.

apps/m3u/tasks.py

@@ -1740,6 +1740,14 @@ def _pick_target_number(
     return _next_available_number(used_numbers, fixed_cursor, end=end_number)
 
 
+def _range_exhausted_error(mode, start_number, end_number, fallback_start):
+    """User-facing range text for RANGE_EXHAUSTED failures."""
+    range_start = (
+        int(fallback_start) if mode == "provider" else int(start_number)
+    )
+    return f"Channel number range {range_start}-{int(end_number)} is full"
+
+
 def _custom_properties_as_dict(value):
     """
     Normalize a JSONField-backed custom_properties value into a dict.
@@ -2454,9 +2462,11 @@ def sync_auto_channels(account_id, scan_start_time=None):
                                     "stream_id": stream.id,
                                     "group": channel_group.name,
                                     "reason": "RANGE_EXHAUSTED",
-                                    "error": (
-                                        f"Channel number range "
-                                        f"{int(start_number)}-{int(end_number)} is full"
+                                    "error": _range_exhausted_error(
+                                        channel_numbering_mode,
+                                        start_number,
+                                        end_number,
+                                        channel_numbering_fallback,
                                     ),
                                 })
                             processed_stream_ids.add(stream.id)

apps/m3u/tests/test_sync_correctness.py

@@ -2494,6 +2494,30 @@ class ProviderNumberingHonorsProviderNumberTests(TestCase):
         manual.refresh_from_db()
         self.assertEqual(manual.channel_number, 88250.0)
 
+    def test_provider_fallback_exhaustion_reports_visible_start(self):
+        # RANGE_EXHAUSTED must cite the fallback range (channel_numbering_fallback
+        # to End), not the hidden auto_sync_channel_start left from another mode.
+        account = _make_account()
+        group = _make_group(name="PPV")
+        rel = _attach_group_to_account(account, group)
+        rel.auto_sync_channel_start = 5000
+        rel.auto_sync_channel_end = 102
+        rel.custom_properties = {
+            "channel_numbering_mode": "provider",
+            "channel_numbering_fallback": 100,
+        }
+        rel.save()
+        for i in range(4):
+            _make_stream(account, group, name=f"S{i}", tvg_id=f"s{i}")
+
+        result = _sync(account)
+
+        self.assertEqual(result["channels_created"], 3)
+        self.assertEqual(result["channels_failed"], 1)
+        error = result["failed_stream_details"][0]["error"]
+        self.assertIn("100-102", error)
+        self.assertNotIn("5000", error)
+
 
 class CrossModeNumberingFieldTests(TestCase):
     """