Add negative and edge-case tests

[dannywillems]

Problem

There are no tests for malformed input, missing required fields, extra unknown fields, empty strings, null values, or boundary integers.

For a schema library, validation behavior on bad input is critical to document and test.

Proposal

Add tests covering at minimum:

• Missing required fields (should raise)
• Extra unknown fields (document behavior: ignore or raise?)
• Null values for required fields
• Empty strings for fields like ip, port
• Boundary integers (0, negative, max int)
• Malformed datetime strings
• Malformed decimal values