Fixed /stock/products/{productId}/add request body parameter validation (fixes #1860)

changelog/68_UNRELEASED_xxxx-xx-xx.md

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+> ⚠️ xxxBREAKING CHANGESxxx
+
+> ❗ xxxImportant upgrade informationXXX
+
+### New feature: xxxx
+
+- xxx
+
+### Stock
+
+- xxx
+
+### Shopping list
+
+- xxx
+
+### Recipes
+
+- xxx
+
+### Meal plan
+
+- xxx
+
+### Chores
+
+- xxx
+
+### Calendar
+
+- xxx
+
+### Tasks
+
+- xxx
+
+### Batteries
+
+- xxx
+
+### Equipment
+
+- xxx
+
+### Userfields
+
+- xxx
+
+### General
+
+- xxx
+
+### API
+
+- Endpoint `/stock/products/{productId}/add`: Fixed that the request body parameter `transaction_type` was ignored / always set to `purchase`