RetrieX shop query meta guard hotfix

This patch hardens the Shopware query optimizer after the language-preservation fix.

Problem

A pure shop command such as suche im shop could be optimized to the meta word shop and then executed as an actual Store API search query.

Adds a configurable shop_prompt.meta_query_guard section in config/retriex/agent.yaml.
Rejects optimized queries that consist only of generic command/meta terms such as shop, suche, im, bitte.
If the current user input is only a shop command, RetrieX tries to use the latest meaningful user question from the recent conversation context as a generic fallback.
If no meaningful context exists, the shop search is skipped and a clear message is shown instead of searching for shop.
Keeps the previous language-preservation behavior for German search terms.

Changed files only:

No retrieval, vector, scoring, prompt-builder, vocabulary, Shopware API or product-family-specific logic was changed.

Run after installing:

php bin/console cache:clear
php bin/console mto:agent:config:validate
php bin/console mto:agent:regression:test