Move regression tests to public Unmarshal API with full error string assertions

Remove the unstable package test and consolidate all test cases into
TestDecodeError_PositionAfterComment, which exercises toml.Unmarshal
and validates the complete human-readable error output including
context lines and tilde markers.

https://claude.ai/code/session_01EXYfFXc3DDGpQ27sWdXTKq

errors_test.go

@@ -310,35 +310,35 @@ func TestDecodeError_PositionAfterComment(t *testing.T) {
 		doc         string
 		expectedRow int
 		expectedCol int
-		errContains string
+		expectedStr string
 	}{
 		{
 			desc:        "invalid key after comment",
 			doc:         "# comment\n= \"value\"",
 			expectedRow: 2,
 			expectedCol: 1,
-			errContains: "invalid character",
+			expectedStr: "1| # comment\n2| = \"value\"\n | ~ invalid character at start of key: =",
 		},
 		{
 			desc:        "invalid key after two comments",
 			doc:         "# one\n# two\n= \"value\"",
 			expectedRow: 3,
 			expectedCol: 1,
-			errContains: "invalid character",
+			expectedStr: "1| # one\n2| # two\n3| = \"value\"\n | ~ invalid character at start of key: =",
 		},
 		{
 			desc:        "invalid key after key-value pair",
 			doc:         "a = 1\n= 2",
 			expectedRow: 2,
 			expectedCol: 1,
-			errContains: "invalid character",
+			expectedStr: "1| a = 1\n2| = 2\n | ~ invalid character at start of key: =",
 		},
 		{
 			desc:        "invalid key after blank line",
 			doc:         "a = 1\n\n= 2",
 			expectedRow: 3,
 			expectedCol: 1,
-			errContains: "invalid character",
+			expectedStr: "1| a = 1\n2|\n3| = 2\n | ~ invalid character at start of key: =",
 		},
 	}
 
@@ -363,9 +363,7 @@ func TestDecodeError_PositionAfterComment(t *testing.T) {
 				t.Errorf("col: got %d, want %d (error: %s)", col, e.expectedCol, derr.String())
 			}
 
-			if !strings.Contains(derr.Error(), e.errContains) {
-				t.Errorf("error %q does not contain %q", derr.Error(), e.errContains)
-			}
+			assert.Equal(t, e.expectedStr, derr.String())
 		})
 	}
 }

unstable/parser_test.go

@@ -695,81 +695,3 @@ func ExampleParser() {
 	// Expression: KeyValue
 	// value -> (Integer) 42
 }
-
-func TestParserError_RangeOffset(t *testing.T) {
-	// Regression test for https://github.com/pelletier/go-toml/issues/1047
-	// Parser.Range must return the correct byte offset for error highlights,
-	// not just for suffix slices.
-	examples := []struct {
-		desc          string
-		input         string
-		wantOffset    int
-		wantLine      int
-		wantColumn    int
-		wantMessage   string
-	}{
-		{
-			desc:        "invalid key start after comment",
-			input:       "# comment\n= \"value\"",
-			wantOffset:  10,
-			wantLine:    2,
-			wantColumn:  1,
-			wantMessage: "invalid character at start of key: =",
-		},
-		{
-			desc:        "invalid key start after two comments",
-			input:       "# one\n# two\n= \"value\"",
-			wantOffset:  12,
-			wantLine:    3,
-			wantColumn:  1,
-			wantMessage: "invalid character at start of key: =",
-		},
-		{
-			desc:        "invalid key start after blank line",
-			input:       "a = 1\n\n= 2",
-			wantOffset:  7,
-			wantLine:    3,
-			wantColumn:  1,
-			wantMessage: "invalid character at start of key: =",
-		},
-		{
-			desc:        "invalid key start after valid key-value",
-			input:       "a = 1\n= 2",
-			wantOffset:  6,
-			wantLine:    2,
-			wantColumn:  1,
-			wantMessage: "invalid character at start of key: =",
-		},
-	}
-
-	for _, e := range examples {
-		t.Run(e.desc, func(t *testing.T) {
-			p := Parser{}
-			p.Reset([]byte(e.input))
-			for p.NextExpression() {
-			}
-			err := p.Error()
-			if err == nil {
-				t.Fatal("expected an error")
-			}
-			perr, ok := err.(*ParserError)
-			if !ok {
-				t.Fatalf("expected *ParserError, got %T", err)
-			}
-
-			assert.Equal(t, e.wantMessage, perr.Message)
-
-			r := p.Range(perr.Highlight)
-			if int(r.Offset) != e.wantOffset {
-				t.Errorf("Range offset: got %d, want %d", r.Offset, e.wantOffset)
-			}
-
-			shape := p.Shape(r)
-			if shape.Start.Line != e.wantLine || shape.Start.Column != e.wantColumn {
-				t.Errorf("position: got %d:%d, want %d:%d",
-					shape.Start.Line, shape.Start.Column,
-					e.wantLine, e.wantColumn)
-			}
-		})
-	}
-}