refactor: share tool schema validation helpers

[atesgoral]

Summary

• Move the shared JSON Schema data-validation flow into MCP::Tool::Schema
• Keep InputSchema#validate_arguments and OutputSchema#validate_result as thin public wrappers with unchanged error messages
• Share schema coercion between the input_schema and output_schema setters

Tests

• dev test test TEST=test/mcp/tool/input_schema_test.rb
• dev test test TEST=test/mcp/tool/output_schema_test.rb
• dev test test TEST=test/mcp/tool_test.rb
• dev test test TEST=test/mcp/server_test.rb
• dev test test TEST=test/mcp/configuration_test.rb
• dev test
• bundle exec rubocop lib/mcp/tool.rb lib/mcp/tool/schema.rb lib/mcp/tool/input_schema.rb lib/mcp/tool/output_schema.rb
• git diff --check

[atesgoral]

This attempt at deduplication wasn't leading to cleaner code or reduction in LOC. Closing.