test(cost): add unit tests for token usage aggregation and precision bounds [skip ci]

tests/test_token_usage_and_cost_bounds.py

@@ -0,0 +1,43 @@
+import pytest
+from typing import Dict, Any
+
+def test_token_usage_aggregation():
+    responses = [
+        {"prompt_tokens": 120, "completion_tokens": 45, "total_tokens": 165},
+        {"prompt_tokens": 80, "completion_tokens": 30, "total_tokens": 110},
+        {"prompt_tokens": 200, "completion_tokens": 90, "total_tokens": 290}
+    ]
+    
+    total_prompt = sum(r["prompt_tokens"] for r in responses)
+    total_completion = sum(r["completion_tokens"] for r in responses)
+    total_combined = sum(r["total_tokens"] for r in responses)
+    
+    assert total_prompt == 400
+    assert total_completion == 165
+    assert total_combined == 565
+    assert total_prompt + total_completion == total_combined
+
+def test_cost_calculation_boundary_zero_tokens():
+    input_cost_per_token = 0.0000015
+    output_cost_per_token = 0.0000020
+    
+    prompt_tokens = 0
+    completion_tokens = 0
+    
+    cost = (prompt_tokens * input_cost_per_token) + (completion_tokens * output_cost_per_token)
+    assert cost == 0.0
+
+def test_cost_calculation_precision():
+    input_cost_per_million = 2.50
+    output_cost_per_million = 10.00
+    
+    prompt_tokens = 100_000
+    completion_tokens = 50_000
+    
+    input_cost = (prompt_tokens / 1_000_000) * input_cost_per_million
+    output_cost = (completion_tokens / 1_000_000) * output_cost_per_million
+    total_cost = round(input_cost + output_cost, 4)
+    
+    assert input_cost == 0.25
+    assert output_cost == 0.50
+    assert total_cost == 0.75