tests: centralize script path setup in conftest.py

Add a pytest-discovered tests/conftest.py for the last30days scripts path and
remove duplicate per-file sys.path.insert boilerplate from tests.

Normalize affected imports to rely on the shared scripts path and remove the
now-unneeded E402 suppressions.

tests/test_providers_v3.py

@@ -1,9 +1,5 @@
 import json
-import sys
 import unittest
-from pathlib import Path
-
-sys.path.insert(0, str(Path(__file__).resolve().parents[1] / "skills" / "last30days" / "scripts"))
 
 from lib import providers
 
@@ -162,6 +158,5 @@ class TestParseCodexStream(unittest.TestCase):
     def test_done_only_stream(self):
         self.assertEqual({}, providers._parse_codex_stream("data: [DONE]\n\n"))
 
-
 if __name__ == "__main__":
     unittest.main()