fix: treat unsuccessful tool results as errors

[one241]

Summary

• treat completed tool outputs with success: false as tool_call_error events
• preserve the tool-provided error or message and use a clear fallback when neither is present
• keep regular objects, strings, and non-boolean success values on the successful completion path

Root cause

The event adapter relied only on the SDK item status. Some tools report a completed SDK item while returning a business-level failure payload, so those failures were emitted as tool_call_complete.

Testing

• pnpm --filter @aipexstudio/aipex-core typecheck
• pnpm --filter @aipexstudio/aipex-core test
• pnpm -r --if-present typecheck
• pnpm -r --if-present test
• pre-push npm run preflight