refactor: Improve code quality, error handling, and test coverage

- Add file_lock context manager to eliminate duplicate locking patterns
- Add ValidationError, ConfigurationError, CertificateError exceptions
- Improve rollback logic in haproxy_add_servers (track successful ops only)
- Decompose haproxy_add_domain into smaller helper functions
- Consolidate certificate constants (CERTS_DIR, ACME_HOME) to config.py
- Enhance docstrings for internal functions and magic numbers
- Add pytest framework with 48 new tests (269 -> 317 total)
- Increase test coverage from 76% to 86%
  - servers.py: 58% -> 82%
  - certificates.py: 67% -> 86%
  - configuration.py: 69% -> 94%

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

tests/unit/test_utils.py

@@ -0,0 +1,130 @@
+"""Unit tests for utils module."""
+
+import pytest
+
+from haproxy_mcp.utils import parse_stat_csv
+
+
+class TestParseStatCsv:
+    """Tests for parse_stat_csv function."""
+
+    def test_parse_valid_csv(self, response_builder):
+        """Parse valid HAProxy stat CSV output."""
+        csv = response_builder.stat_csv([
+            {"pxname": "pool_1", "svname": "pool_1_1", "scur": 5, "status": "UP", "weight": 1, "check_status": "L4OK"},
+            {"pxname": "pool_1", "svname": "pool_1_2", "scur": 3, "status": "UP", "weight": 1, "check_status": "L4OK"},
+        ])
+
+        results = list(parse_stat_csv(csv))
+
+        assert len(results) == 2
+        assert results[0]["pxname"] == "pool_1"
+        assert results[0]["svname"] == "pool_1_1"
+        assert results[0]["scur"] == "5"
+        assert results[0]["status"] == "UP"
+        assert results[0]["weight"] == "1"
+        assert results[0]["check_status"] == "L4OK"
+
+    def test_parse_empty_output(self):
+        """Parse empty output returns no results."""
+        results = list(parse_stat_csv(""))
+        assert results == []
+
+    def test_parse_header_only(self):
+        """Parse output with only header returns no results."""
+        csv = "# pxname,svname,qcur,qmax,scur,smax,..."
+        results = list(parse_stat_csv(csv))
+        assert results == []
+
+    def test_skip_comment_lines(self):
+        """Comment lines are skipped."""
+        csv = """# This is a comment
+# Another comment
+pool_1,pool_1_1,0,0,5,10,0,0,0,0,0,0,0,0,0,0,0,UP,1,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,L4OK,"""
+
+        results = list(parse_stat_csv(csv))
+        assert len(results) == 1
+        assert results[0]["pxname"] == "pool_1"
+
+    def test_skip_empty_lines(self):
+        """Empty lines are skipped."""
+        csv = """
+pool_1,pool_1_1,0,0,5,10,0,0,0,0,0,0,0,0,0,0,0,UP,1,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,L4OK,
+
+pool_1,pool_1_2,0,0,3,10,0,0,0,0,0,0,0,0,0,0,0,UP,1,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,L4OK,
+"""
+
+        results = list(parse_stat_csv(csv))
+        assert len(results) == 2
+
+    def test_parse_down_status(self, response_builder):
+        """Parse server with DOWN status."""
+        csv = response_builder.stat_csv([
+            {"pxname": "pool_1", "svname": "pool_1_1", "status": "DOWN", "check_status": "L4TOUT"},
+        ])
+
+        results = list(parse_stat_csv(csv))
+
+        assert len(results) == 1
+        assert results[0]["status"] == "DOWN"
+        assert results[0]["check_status"] == "L4TOUT"
+
+    def test_parse_maint_status(self, response_builder):
+        """Parse server with MAINT status."""
+        csv = response_builder.stat_csv([
+            {"pxname": "pool_1", "svname": "pool_1_1", "status": "MAINT"},
+        ])
+
+        results = list(parse_stat_csv(csv))
+
+        assert len(results) == 1
+        assert results[0]["status"] == "MAINT"
+
+    def test_parse_multiple_backends(self, response_builder):
+        """Parse output with multiple backends."""
+        csv = response_builder.stat_csv([
+            {"pxname": "pool_1", "svname": "pool_1_1", "status": "UP"},
+            {"pxname": "pool_2", "svname": "pool_2_1", "status": "UP"},
+            {"pxname": "pool_3", "svname": "pool_3_1", "status": "DOWN"},
+        ])
+
+        results = list(parse_stat_csv(csv))
+
+        assert len(results) == 3
+        assert results[0]["pxname"] == "pool_1"
+        assert results[1]["pxname"] == "pool_2"
+        assert results[2]["pxname"] == "pool_3"
+
+    def test_parse_frontend_backend_rows(self):
+        """Frontend and BACKEND rows are included."""
+        csv = """pool_1,FRONTEND,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,UP,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,,
+pool_1,pool_1_1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,UP,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,L4OK,
+pool_1,BACKEND,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,UP,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,,"""
+
+        results = list(parse_stat_csv(csv))
+
+        # All rows with enough columns are returned
+        assert len(results) == 3
+        svnames = [r["svname"] for r in results]
+        assert "FRONTEND" in svnames
+        assert "pool_1_1" in svnames
+        assert "BACKEND" in svnames
+
+    def test_parse_insufficient_columns(self):
+        """Rows with insufficient columns are skipped."""
+        csv = "pool_1,pool_1_1,0,0,5"  # Only 5 columns, need more than 17
+
+        results = list(parse_stat_csv(csv))
+        assert results == []
+
+    def test_generator_is_lazy(self, response_builder):
+        """Verify parse_stat_csv returns a generator (lazy evaluation)."""
+        csv = response_builder.stat_csv([
+            {"pxname": "pool_1", "svname": "pool_1_1", "status": "UP"},
+        ])
+
+        result = parse_stat_csv(csv)
+
+        # Should return a generator, not a list
+        import types
+        assert isinstance(result, types.GeneratorType)