test: add comprehensive tests for trailing whitespace rule

- Test all check scenarios: no violations, single/multiple violations, edge cases
- Test auto-fix functionality with various whitespace types
- Test rule metadata (ID, name, severity, canAutoFix)
- All tests passing, addresses critical test coverage gap from PR #111 review

Author: Ajit Pratap Singh

pkg/linter/rules/whitespace/trailing_whitespace_test.go

@@ -0,0 +1,162 @@
+package whitespace
+
+import (
+	"testing"
+
+	"github.com/ajitpratap0/GoSQLX/pkg/linter"
+)
+
+func TestTrailingWhitespaceRule_Check(t *testing.T) {
+	tests := []struct {
+		name               string
+		sql                string
+		expectedViolations int
+	}{
+		{
+			name:               "No trailing whitespace",
+			sql:                "SELECT id, name\nFROM users\nWHERE active = true",
+			expectedViolations: 0,
+		},
+		{
+			name:               "Single line with trailing whitespace",
+			sql:                "SELECT id   ",
+			expectedViolations: 1,
+		},
+		{
+			name:               "Multiple lines with trailing whitespace",
+			sql:                "SELECT id   \nFROM users  \nWHERE active = true   ",
+			expectedViolations: 3,
+		},
+		{
+			name:               "Empty lines should be skipped",
+			sql:                "SELECT id\n\nFROM users",
+			expectedViolations: 0,
+		},
+		{
+			name:               "Tab as trailing whitespace",
+			sql:                "SELECT id\t",
+			expectedViolations: 1,
+		},
+		{
+			name:               "Mixed spaces and tabs as trailing",
+			sql:                "SELECT id  \t  ",
+			expectedViolations: 1,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			rule := NewTrailingWhitespaceRule()
+			ctx := linter.NewContext(tt.sql, "test.sql")
+
+			violations, err := rule.Check(ctx)
+			if err != nil {
+				t.Fatalf("Unexpected error: %v", err)
+			}
+
+			if len(violations) != tt.expectedViolations {
+				t.Errorf("Expected %d violations, got %d", tt.expectedViolations, len(violations))
+				for i, v := range violations {
+					t.Logf("Violation %d: %s at line %d", i+1, v.Message, v.Location.Line)
+				}
+			}
+
+			// Verify violation details
+			for _, v := range violations {
+				if v.Rule != "L001" {
+					t.Errorf("Expected rule ID 'L001', got '%s'", v.Rule)
+				}
+				if v.Severity != linter.SeverityWarning {
+					t.Errorf("Expected severity 'warning', got '%s'", v.Severity)
+				}
+				if !v.CanAutoFix {
+					t.Error("Expected CanAutoFix to be true")
+				}
+			}
+		})
+	}
+}
+
+func TestTrailingWhitespaceRule_Fix(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected string
+	}{
+		{
+			name:     "Remove trailing spaces",
+			input:    "SELECT id   \nFROM users",
+			expected: "SELECT id\nFROM users",
+		},
+		{
+			name:     "Remove trailing tabs",
+			input:    "SELECT id\t\t\nFROM users",
+			expected: "SELECT id\nFROM users",
+		},
+		{
+			name:     "Remove mixed trailing whitespace",
+			input:    "SELECT id  \t  \nFROM users",
+			expected: "SELECT id\nFROM users",
+		},
+		{
+			name:     "Preserve lines without trailing whitespace",
+			input:    "SELECT id\nFROM users",
+			expected: "SELECT id\nFROM users",
+		},
+		{
+			name:     "Handle empty lines",
+			input:    "SELECT id\n\nFROM users",
+			expected: "SELECT id\n\nFROM users",
+		},
+		{
+			name:     "Multiple lines with trailing whitespace",
+			input:    "SELECT id   \nFROM users  \nWHERE active = true   ",
+			expected: "SELECT id\nFROM users\nWHERE active = true",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			rule := NewTrailingWhitespaceRule()
+			ctx := linter.NewContext(tt.input, "test.sql")
+
+			violations, err := rule.Check(ctx)
+			if err != nil {
+				t.Fatalf("Unexpected error during check: %v", err)
+			}
+
+			fixed, err := rule.Fix(tt.input, violations)
+			if err != nil {
+				t.Fatalf("Unexpected error during fix: %v", err)
+			}
+
+			if fixed != tt.expected {
+				t.Errorf("Fix result mismatch:\nExpected: %q\nGot:      %q", tt.expected, fixed)
+			}
+		})
+	}
+}
+
+func TestTrailingWhitespaceRule_Metadata(t *testing.T) {
+	rule := NewTrailingWhitespaceRule()
+
+	if rule.ID() != "L001" {
+		t.Errorf("Expected ID 'L001', got '%s'", rule.ID())
+	}
+
+	if rule.Name() != "Trailing Whitespace" {
+		t.Errorf("Expected name 'Trailing Whitespace', got '%s'", rule.Name())
+	}
+
+	if rule.Severity() != linter.SeverityWarning {
+		t.Errorf("Expected severity 'warning', got '%s'", rule.Severity())
+	}
+
+	if !rule.CanAutoFix() {
+		t.Error("Expected CanAutoFix to be true")
+	}
+
+	if rule.Description() == "" {
+		t.Error("Expected non-empty description")
+	}
+}