Fix user response cache type error

[jeffbyrnes]

Summary

This fixes the Cannot assign array to property Search::$user of type stdClass fatal seen when invoking the workflow with a cached /user response that is no longer an object.

Compared to #164, this keeps the local variable safety improvement but also:

• verifies the fetched /user response is actually a stdClass before assigning it to Search::$user
• clears a bad cached /user row so the workflow can recover instead of repeatedly failing
• hardens Fetcher so object responses are not merged with paginated child cache rows
• removes stale child cache rows when an object response is refreshed
• adds regression coverage for corrupt child-cache rows attached to object endpoints

This is intended to supersede #164.

Testing

• php -l src/fetcher.php
• php -l src/search.php
• php -l tests/FetcherTest.php

I could not run the PHPUnit suite locally because composer / vendor are not installed in this checkout.

AI assistance disclosure

These changes were generated by an AI coding assistant at the request of @jeffbyrnes.

• Assistant: OpenAI ChatGPT, operating in the Pi coding-agent harness
• Provider: openai-codex
• Model: gpt-5.5