fix: signal ask_user execution failures

[MDGChamomile]

Summary

• throw for malformed options, unavailable UI, and UI execution failures
• render thrown tool failures using Pi's renderer error context
• update regression tests and the changelog

Why

Pi only marks a custom tool execution as failed when execute() throws. Returning an object with isError: true does not set the tool result error flag, so these three ask_user failure paths were emitted as successful executions.

See Pi's custom tool error contract.

Verification

• tsc --noEmit --strict --skipLibCheck --module esnext --moduleResolution bundler --target es2022 index.ts
• test sources type-check with Bun types
• targeted runtime smoke checks for all three rejection paths, lifecycle cleanup, and error rendering
• npm pack --dry-run

The full Bun test suite was not run locally because Bun is unavailable in this environment.