Add comprehensive tests for CostTracker class

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Co-authored-by: abdoelsayed2016 <27821589+abdoelsayed2016@users.noreply.github.com>

tests/test_cost_tracker.py

@@ -0,0 +1,52 @@
+from __future__ import annotations
+
+import unittest
+
+from src.cost_tracker import CostTracker
+
+
+class CostTrackerTests(unittest.TestCase):
+    def test_fresh_tracker_starts_at_zero_with_empty_events(self) -> None:
+        tracker = CostTracker()
+        self.assertEqual(tracker.total_units, 0)
+        self.assertEqual(tracker.events, [])
+
+    def test_record_single_event(self) -> None:
+        tracker = CostTracker()
+        tracker.record('inference', 42)
+        self.assertEqual(tracker.total_units, 42)
+        self.assertEqual(len(tracker.events), 1)
+
+    def test_record_multiple_events_accumulates_totals(self) -> None:
+        tracker = CostTracker()
+        tracker.record('inference', 10)
+        tracker.record('embedding', 20)
+        tracker.record('search', 30)
+        self.assertEqual(tracker.total_units, 60)
+        self.assertEqual(len(tracker.events), 3)
+
+    def test_record_zero_units(self) -> None:
+        tracker = CostTracker()
+        tracker.record('noop', 0)
+        self.assertEqual(tracker.total_units, 0)
+        self.assertEqual(len(tracker.events), 1)
+        self.assertIn('noop:0', tracker.events)
+
+    def test_record_large_units(self) -> None:
+        tracker = CostTracker()
+        large = 10**9
+        tracker.record('bulk', large)
+        self.assertEqual(tracker.total_units, large)
+        self.assertEqual(tracker.events, [f'bulk:{large}'])
+
+    def test_event_format_is_label_colon_units(self) -> None:
+        tracker = CostTracker()
+        tracker.record('inference', 42)
+        self.assertEqual(tracker.events[0], 'inference:42')
+
+    def test_events_are_ordered_chronologically(self) -> None:
+        tracker = CostTracker()
+        labels = ['first', 'second', 'third']
+        for i, label in enumerate(labels):
+            tracker.record(label, i)
+        self.assertEqual(tracker.events, ['first:0', 'second:1', 'third:2'])