feat: add validator error cases and twitter.com support

Added comprehensive test coverage for URL validation:
- Test for invalid URL formats (missing /i/, missing space ID, malformed URLs)
- Test for twitter.com domain support (in addition to x.com)

Updated validator implementation:
- Extended regex pattern to accept both x.com and twitter.com domains
- Updated group capture index to account for domain alternation

All validator tests passing (3/3).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

tests/test_validator.py

@@ -10,3 +10,29 @@ def test_validate_space_url_valid():
     assert is_valid is True
     assert space_id == "1ZkKzZWnVRRKv"
     assert error is None
+
+
+def test_validate_space_url_invalid_format():
+    """Test that invalid URLs are rejected."""
+    invalid_urls = [
+        "https://x.com/spaces/1ZkKzZWnVRRKv",  # Missing /i/
+        "https://x.com/i/spaces/",  # Missing space ID
+        "not a url",  # Completely invalid
+        "",  # Empty string
+    ]
+
+    for url in invalid_urls:
+        is_valid, space_id, error = validate_space_url(url)
+        assert is_valid is False
+        assert space_id is None
+        assert "Invalid URL format" in error
+
+
+def test_validate_space_url_twitter_domain():
+    """Test that twitter.com URLs are also accepted."""
+    url = "https://twitter.com/i/spaces/1ZkKzZWnVRRKv"
+    is_valid, space_id, error = validate_space_url(url)
+
+    assert is_valid is True
+    assert space_id == "1ZkKzZWnVRRKv"
+    assert error is None