Create unit tests for the PromptCodeBlockExtractor

Problem/Motivation

Currently there is no testing that the output of the PromptCodeBlockExtractor service is actually correct, we need to have extensive testing with multiple different strings being tested.

See the testing for PromptJsonDecoderTest for ideas.

Steps to reproduce

Proposed resolution

Add tests and test things like having ```html inside a text, having it start with
Also do the same for yaml, json, css
Test it with streaming

Remaining tasks

User interface changes

API changes

Data model changes