Fix: Resolve Agentic failure loop with improved json parsing

[ahgraber]

Fixes #1538

• Add final instruction for better json schema adherence and reduce "filler" text
• Add extract_json function to identify json by pairs of [] or {}
• Add unit tests for extract_json

Notes:
In my testing, these changes vastly reduce the number of times the 'repair' agent is triggered. However, this does not fix all parse errors:

1. In some cases, json is valid json, but keys are invalid
2. In some cases, especially with more complex Pydantic models (e.g., Themes, which is composed of List[Theme]), the Pydantic json schema includes $defs. As a result, some LLMs will return $defs or include "$schema": "https://json-schema.org/draft/...; either of these additional keys will cause the Pydantic parser to fail.

[jjmachan]

thanks a lot @ahgraber for this PR ❤️
will be releasing it today