Add unit tests for Pip.indices() config parsing

Regression test for the literal \n splitting fix plus coverage for
single index, extra-index, multiple-index priority, empty config,
and config call failure scenarios.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: harkrish-1

test/unit/test_pip_indices.py

@@ -0,0 +1,66 @@
+from unittest.mock import patch, MagicMock
+
+from metaflow.plugins.pypi.pip import Pip
+
+
+def _make_pip():
+    pip = object.__new__(Pip)
+    return pip
+
+
+class TestPipIndices:
+    def test_single_index_url(self):
+        pip = _make_pip()
+        config_output = "global.index-url='https://pypi.org/simple'"
+        with patch.object(pip, "_call", return_value=config_output):
+            index, extras = pip.indices("dummy")
+        assert index == "https://pypi.org/simple"
+        assert extras == []
+
+    def test_single_extra_index_url(self):
+        pip = _make_pip()
+        config_output = (
+            "global.index-url='https://pypi.org/simple'\n"
+            "global.extra-index-url='https://extra.example.com/simple'"
+        )
+        with patch.object(pip, "_call", return_value=config_output):
+            index, extras = pip.indices("dummy")
+        assert index == "https://pypi.org/simple"
+        assert extras == ["https://extra.example.com/simple"]
+
+    def test_multiple_extra_index_urls_literal_newline(self):
+        """Regression test: pip config list separates multiple URLs with literal \\n."""
+        pip = _make_pip()
+        config_output = (
+            "global.index-url='https://pypi.org/simple'\n"
+            r"global.extra-index-url='https://extra1.example.com/simple'\n'https://extra2.example.com/simple'"
+        )
+        with patch.object(pip, "_call", return_value=config_output):
+            index, extras = pip.indices("dummy")
+        assert index == "https://pypi.org/simple"
+        assert extras == [
+            "https://extra1.example.com/simple",
+            "https://extra2.example.com/simple",
+        ]
+
+    def test_multiple_index_urls_first_wins(self):
+        pip = _make_pip()
+        config_output = r"global.index-url='https://first.example.com'\n'https://second.example.com'"
+        with patch.object(pip, "_call", return_value=config_output):
+            index, extras = pip.indices("dummy")
+        assert index == "https://first.example.com"
+        assert extras == ["https://second.example.com"]
+
+    def test_empty_config(self):
+        pip = _make_pip()
+        with patch.object(pip, "_call", return_value=""):
+            index, extras = pip.indices("dummy")
+        assert index is None
+        assert extras == []
+
+    def test_config_call_fails(self):
+        pip = _make_pip()
+        with patch.object(pip, "_call", side_effect=Exception("pip not found")):
+            index, extras = pip.indices("dummy")
+        assert index is None
+        assert extras == []