Refactor schema validation to reduce duplication and add additionalProperties support

[ebeigarts]

This refactoring extracts common schema validation logic into a base Schema class, eliminating ~100 lines of duplicate code between InputSchema and OutputSchema. It also adds support for the JSON Schema additionalProperties keyword to allow fine-grained control over extra properties in tool arguments.

Previously additionalParameters could be set only on OutputSchema.

Key changes:

• Created MCP::Tool::Schema base class consolidating:

  • Schema validation against JSON Schema Draft 4 metaschema
  • Deep key transformation (string to symbol conversion)
  • $ref disallowance checking
  • Common validation methods

• Refactored InputSchema to inherit from Schema:

  • Simplified constructor to accept full schema hash
  • Changed required fields storage from symbols to strings
  • Removed ~40 lines of duplicated validation logic

• Refactored OutputSchema to inherit from Schema:

  • Removed ~54 lines of duplicated validation logic
  • Now shares all common functionality with InputSchema

• Updated Tool.input_schema setter:

  • Simplified to pass full hash directly to InputSchema

• Added additionalProperties support:

  • By default, tools now allow additional properties (JSON Schema default)
  • Can explicitly set additionalProperties: false to disallow extras
  • Added comprehensive test coverage for both behaviors

Breaking Changes

InputSchema doesn't have required and properties readers.
Required params are now strings instead of symbols to match OutputSchema.

Before:

input_schema.required   # => [:message, :name]
input_schema.properties # => { ... }

After:

input_schema.schema[:required]   # => ["message", "name"]
input_schema.schema[:properties] # => { ... }

Error messages are unified:

Invalid JSON Schema: $ref is not allowed in tool schemas

instead of

Invalid JSON Schema: $ref is not allowed in tool input schemas
Invalid JSON Schema: $ref is not allowed in tool output schemas

Impact:

• Direct access to properties attribute removed
• Direct access to required attribute removed
• Must use schema hash or to_h method instead