Add comprehensive error handling and edge case tests

New test suite test_errors.mojo with 11 tests:
- Error detection: unclosed sections, duplicate keys, missing equals
- Edge cases: empty sections, special characters, Unicode
- Stress tests: very long lines (10K chars), many sections (100+)
- Robustness: empty files, comment-only files, quoted section names

All tests verify proper error messages and graceful handling.
Total: 46 tests passing (35 original + 11 error tests)

Co-Authored-By: Warp <agent@warp.dev>

Author: Mjboothaus

tests/test_errors.mojo

@@ -0,0 +1,144 @@
+"""Tests for error handling and edge cases.
+
+These tests verify that the parser properly detects and reports errors
+for invalid INI syntax, matching Python configparser behavior where appropriate.
+"""
+
+from testing import assert_true, TestSuite
+from ini.parser import parse
+
+
+fn test_unclosed_section_header() raises:
+    """Test that unclosed section header raises error."""
+    var input = "[Section"
+
+    try:
+        var _ = parse(input)
+        assert_true(False, "Should raise error for unclosed section")
+    except e:
+        # Expected - should contain "Unclosed"
+        var err_msg = String(e)
+        assert_true("Unclosed" in err_msg, "Error should mention unclosed section")
+
+
+fn test_duplicate_key_in_section() raises:
+    """Test that duplicate keys in same section raise error."""
+    var input = """[Test]
+key = value1
+key = value2"""
+
+    try:
+        var _ = parse(input)
+        assert_true(False, "Should raise error for duplicate key")
+    except e:
+        # Expected - should mention "Duplicate"
+        var err_msg = String(e)
+        assert_true("Duplicate" in err_msg, "Error should mention duplicate key")
+
+
+fn test_missing_equals_sign() raises:
+    """Test that missing equals sign raises error."""
+    var input = """[Test]
+key_without_equals"""
+
+    try:
+        var _ = parse(input)
+        assert_true(False, "Should raise error for missing equals")
+    except e:
+        # Expected - should mention expecting '='
+        var err_msg = String(e)
+        assert_true("=" in err_msg or "Expected" in err_msg, "Error should mention missing =")
+
+
+fn test_empty_section_name() raises:
+    """Test that empty section name is handled."""
+    var input = "[]\nkey = value"
+
+    # Python configparser allows empty section names
+    var config = parse(input)
+    # Just check that it doesn't crash - empty string key should exist
+    assert_true("" in config, "Should handle empty section name")
+
+
+fn test_special_characters_in_values() raises:
+    """Test that special characters in values are preserved."""
+    var input = """[Test]
+url = https://example.com/path?query=value&other=123
+path = /path/with spaces/file.txt
+email = user@example.com
+brackets = [not a section]
+equals = key=value inside value"""
+
+    var config = parse(input)
+    assert_true("?" in config["Test"]["url"], "Should preserve ? in URL")
+    assert_true("&" in config["Test"]["url"], "Should preserve & in URL")
+    assert_true(" " in config["Test"]["path"], "Should preserve spaces in path")
+    assert_true("@" in config["Test"]["email"], "Should preserve @ in email")
+    assert_true("[" in config["Test"]["brackets"], "Should preserve [ in value")
+    assert_true("=" in config["Test"]["equals"], "Should preserve = in value")
+
+
+fn test_unicode_values() raises:
+    """Test that Unicode characters are handled correctly."""
+    var input = """[Test]
+greeting = Hello, 世界!
+emoji = 🔥 mojo-ini 🚀
+cyrillic = Привет мир"""
+
+    var config = parse(input)
+    assert_true("世界" in config["Test"]["greeting"], "Should handle Chinese characters")
+    assert_true("🔥" in config["Test"]["emoji"], "Should handle emoji")
+    assert_true("Привет" in config["Test"]["cyrillic"], "Should handle Cyrillic")
+
+
+fn test_very_long_lines() raises:
+    """Test that very long lines are handled."""
+    var long_value = "x" * 10000
+    var input = "[Test]\nkey = " + long_value
+
+    var config = parse(input)
+    assert_true(len(config["Test"]["key"]) == 10000, "Should handle very long values")
+
+
+fn test_many_sections() raises:
+    """Test that many sections can be parsed."""
+    var input = String("")
+    for i in range(100):
+        input += "[Section" + String(i) + "]\n"
+        input += "key = value" + String(i) + "\n"
+
+    var config = parse(input)
+    # Should have 100 sections + default section
+    assert_true(len(config) >= 100, "Should handle many sections (got " + String(len(config)) + ")")
+
+
+fn test_empty_file() raises:
+    """Test that empty file is handled gracefully."""
+    var config = parse("")
+    # Should have at least the default section
+    assert_true("" in config, "Empty file should have default section")
+
+
+fn test_only_comments() raises:
+    """Test that file with only comments is handled."""
+    var input = """# Comment 1
+; Comment 2
+# Comment 3"""
+
+    var config = parse(input)
+    assert_true("" in config, "Comment-only file should have default section")
+
+
+fn test_section_with_quotes() raises:
+    """Test section names with quotes (Git config style)."""
+    var input = """[remote "origin"]
+url = https://github.com/user/repo.git"""
+
+    var config = parse(input)
+    # Section name should include the quotes
+    assert_true('remote "origin"' in config, "Should preserve quotes in section name")
+
+
+fn main() raises:
+    """Run all error handling tests using TestSuite."""
+    TestSuite.discover_tests[__functions_in_module()]().run()