test: fix _ext import (only exists in ab_ai_mail, not receipt_parser)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/test_expenses_agent.py

@@ -399,15 +399,10 @@ async def test_act_no_employee_returns_empty_and_escalates():
 # parse_upload — receipt_parser.py
 # ---------------------------------------------------------------------------
 
-from agent_service.tools.receipt_parser import parse_upload, _ext
+from agent_service.tools.receipt_parser import parse_upload
 
 
 class TestParseUpload:
-    def test_ext_helper(self):
-        assert _ext('receipt.JPG') == 'jpg'
-        assert _ext('file.tar.gz') == 'gz'
-        assert _ext('noextension') == ''
-
     def test_text_file_parsed(self):
         results = parse_upload('receipt.txt', b'Acme Store\nTotal: $10.00')
         assert len(results) == 1