Function validate_tool_definition 

pub fn validate_tool_definition() -> ToolDefinition

Return the MCP tool definition for apr.validate.

FALSIFY-MCP-008: the inputSchema is parsed from the build-time codegen constant crate::schemas::APR_VALIDATE_SCHEMA, which build.rs emits from contracts/apr-mcp-tool-schemas-v1.yaml. The contract is the single source of truth — the live tools/list response and the YAML must agree byte-for-byte after JSON canonicalization (asserted by tests/falsify_mcp_008.rs).