test(backend): cover usd_to_microdollars(None) and get_platform_cost_logs with explicit start

Closes branch gaps in platform_cost.py (lines 29-31 and 312→314) that
were introduced via the dev merge but not exercised by existing tests.
This also forces the backend CI to run so Codecov uploads fresh coverage
instead of carrying forward stale data from before the cost-tracking
feature landed on dev.

autogpt_platform/backend/backend/data/platform_cost_test.py

@@ -16,9 +16,21 @@ from .platform_cost import (
     get_platform_cost_logs,
     log_platform_cost,
     log_platform_cost_safe,
+    usd_to_microdollars,
 )
 
 
+class TestUsdToMicrodollars:
+    def test_none_returns_none(self):
+        assert usd_to_microdollars(None) is None
+
+    def test_converts_usd_to_microdollars(self):
+        assert usd_to_microdollars(1.0) == 1_000_000
+
+    def test_fractional_usd(self):
+        assert usd_to_microdollars(0.0042) == 4200
+
+
 class TestMaskEmail:
     def test_typical_email(self):
         assert _mask_email("user@example.com") == "us***@example.com"
@@ -284,3 +296,13 @@ class TestGetPlatformCostLogs:
         with patch("backend.data.platform_cost.query_raw_with_schema", new=mock_query):
             logs, total = await get_platform_cost_logs()
         assert total == 0
+
+    @pytest.mark.asyncio
+    async def test_explicit_start_skips_default(self):
+        start = datetime(2026, 1, 1, tzinfo=timezone.utc)
+        mock_query = AsyncMock(side_effect=[[{"cnt": 0}], []])
+        with patch("backend.data.platform_cost.query_raw_with_schema", new=mock_query):
+            logs, total = await get_platform_cost_logs(start=start)
+        assert total == 0
+        first_call_sql = mock_query.call_args_list[0][0][0]
+        assert "createdAt" in first_call_sql