refactor: improve Vertex AI model conversion and add tests

- Rename compute_mode -> convert_model_for_vertex for clarity
- Move `import re` to module top-level (stdlib convention)
- Use greedy regex quantifier for more readable pattern matching
- Restore PEP 8 double blank line between top-level definitions
- Add test_client.py with 10 unit tests covering:
  - Vertex disabled (env unset, "0", empty)
  - Standard conversions (Opus, Sonnet, Haiku)
  - Edge cases (already-converted, non-Claude, no date suffix, empty)

Follow-up improvements from PR #129 review.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

test_client.py

@@ -0,0 +1,105 @@
+#!/usr/bin/env python3
+"""
+Client Utility Tests
+====================
+
+Tests for the client module utility functions.
+Run with: python test_client.py
+"""
+
+import os
+import unittest
+
+from client import convert_model_for_vertex
+
+
+class TestConvertModelForVertex(unittest.TestCase):
+    """Tests for convert_model_for_vertex function."""
+
+    def setUp(self):
+        """Save original env state."""
+        self._orig_vertex = os.environ.get("CLAUDE_CODE_USE_VERTEX")
+
+    def tearDown(self):
+        """Restore original env state."""
+        if self._orig_vertex is None:
+            os.environ.pop("CLAUDE_CODE_USE_VERTEX", None)
+        else:
+            os.environ["CLAUDE_CODE_USE_VERTEX"] = self._orig_vertex
+
+    # --- Vertex AI disabled (default) ---
+
+    def test_returns_model_unchanged_when_vertex_disabled(self):
+        os.environ.pop("CLAUDE_CODE_USE_VERTEX", None)
+        self.assertEqual(
+            convert_model_for_vertex("claude-opus-4-5-20251101"),
+            "claude-opus-4-5-20251101",
+        )
+
+    def test_returns_model_unchanged_when_vertex_set_to_zero(self):
+        os.environ["CLAUDE_CODE_USE_VERTEX"] = "0"
+        self.assertEqual(
+            convert_model_for_vertex("claude-opus-4-5-20251101"),
+            "claude-opus-4-5-20251101",
+        )
+
+    def test_returns_model_unchanged_when_vertex_set_to_empty(self):
+        os.environ["CLAUDE_CODE_USE_VERTEX"] = ""
+        self.assertEqual(
+            convert_model_for_vertex("claude-sonnet-4-5-20250929"),
+            "claude-sonnet-4-5-20250929",
+        )
+
+    # --- Vertex AI enabled: standard conversions ---
+
+    def test_converts_opus_model(self):
+        os.environ["CLAUDE_CODE_USE_VERTEX"] = "1"
+        self.assertEqual(
+            convert_model_for_vertex("claude-opus-4-5-20251101"),
+            "claude-opus-4-5@20251101",
+        )
+
+    def test_converts_sonnet_model(self):
+        os.environ["CLAUDE_CODE_USE_VERTEX"] = "1"
+        self.assertEqual(
+            convert_model_for_vertex("claude-sonnet-4-5-20250929"),
+            "claude-sonnet-4-5@20250929",
+        )
+
+    def test_converts_haiku_model(self):
+        os.environ["CLAUDE_CODE_USE_VERTEX"] = "1"
+        self.assertEqual(
+            convert_model_for_vertex("claude-3-5-haiku-20241022"),
+            "claude-3-5-haiku@20241022",
+        )
+
+    # --- Vertex AI enabled: already converted or non-matching ---
+
+    def test_already_vertex_format_unchanged(self):
+        os.environ["CLAUDE_CODE_USE_VERTEX"] = "1"
+        self.assertEqual(
+            convert_model_for_vertex("claude-opus-4-5@20251101"),
+            "claude-opus-4-5@20251101",
+        )
+
+    def test_non_claude_model_unchanged(self):
+        os.environ["CLAUDE_CODE_USE_VERTEX"] = "1"
+        self.assertEqual(
+            convert_model_for_vertex("gpt-4o"),
+            "gpt-4o",
+        )
+
+    def test_model_without_date_suffix_unchanged(self):
+        os.environ["CLAUDE_CODE_USE_VERTEX"] = "1"
+        self.assertEqual(
+            convert_model_for_vertex("claude-opus-4-5"),
+            "claude-opus-4-5",
+        )
+
+    def test_empty_string_unchanged(self):
+        os.environ["CLAUDE_CODE_USE_VERTEX"] = "1"
+        self.assertEqual(convert_model_for_vertex(""), "")
+
+
+if __name__ == "__main__":
+    unittest.main()