fix: allow additional properties by default per JSON Schema spec (#617)

- Remove automatic additionalProperties: false injection (JSON Schema spec compliance)
- Support String input for structuredContent in validate() method
- Move tests to mcp-json-jackson2 module with proper dependencies
- Replace wildcard imports with explicit imports

Complies with JSON Schema Test Suite:
https://github.com/json-schema-org/JSON-Schema-Test-Suite/blob/15e4505/tests/draft2020-12/additionalProperties.json\#L112



Fixes #584

BREAKING CHANGE: Additional properties now allowed by default when not explicitly specified.

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>