feat: add opt-in tool output validation

[atesgoral]

Summary

• Add validate_tool_call_results, disabled by default, to opt into server-side output schema validation
• Validate successful tool structuredContent against output_schema when the option is enabled
• Treat invalid or missing structuredContent as an internal tool error and skip validation for error responses
• Document the new option and cover default, valid, invalid, missing, and error-response behavior

Stacked on #343. This PR currently includes #343 because GitHub cannot base an upstream PR on a fork-only branch; after #343 merges, the diff should collapse to the output-validation commit.

Tests

• dev test test TEST=test/mcp/configuration_test.rb
• dev test test TEST=test/mcp/server_test.rb
• dev test test TEST=test/mcp/tool/output_schema_test.rb
• dev test test TEST=test/mcp/client_test.rb
• dev test
• git diff --check

Note

• Targeted RuboCop on touched Ruby files still reports the existing Minitest/AssertEmptyLiteral offense at test/mcp/server_test.rb:713.