test(core): add error reporting tests

src/test/kotlin/lumen/ErrorReportTest.kt

@@ -0,0 +1,48 @@
+package lumen
+
+import kotlin.test.Test
+import kotlin.test.assertEquals
+import kotlin.test.assertFailsWith
+import kotlin.test.assertTrue
+
+class ErrorReportTest {
+    @Test
+    fun lexErrorCarriesLineAndColumn() {
+        val error = assertFailsWith<LexError> { Lumen.tokenize("let x = @") }
+        assertEquals(1, error.line)
+        assertEquals(9, error.column)
+    }
+
+    @Test
+    fun parseErrorCarriesLineAndColumn() {
+        val error = assertFailsWith<ParseError> { Lumen.parse("println(1 +)") }
+        assertEquals(1, error.line)
+        assertEquals(12, error.column)
+    }
+
+    @Test
+    fun runtimeErrorCarriesLineAndColumn() {
+        val error = assertFailsWith<RuntimeError> {
+            Lumen.run("let xs = [1]\nprintln(xs[5])", StringBuilder())
+        }
+        assertEquals(2, error.line)
+        assertTrue(error.column > 0, "runtime error should report a column")
+    }
+
+    @Test
+    fun errorOnSecondLineReportsSecondLine() {
+        val error = assertFailsWith<LexError> { Lumen.tokenize("let x = 1\nlet y = @") }
+        assertEquals(2, error.line)
+        assertEquals(9, error.column)
+    }
+
+    @Test
+    fun formatErrorRendersSourceFragmentWithCaret() {
+        val source = "let x = 1\nlet y = @"
+        val error = assertFailsWith<LexError> { Lumen.tokenize(source) }
+        val report = formatError(source, error)
+        assertTrue(report.contains("line 2, column 9"), "report should name the position")
+        assertTrue(report.contains("let y = @"), "report should quote the offending source line")
+        assertTrue(report.contains("^"), "report should point at the column with a caret")
+    }
+}