Support exporting PyDough metadata graph to JSON format #543

It is possible to load a JSON metadata graph into PyDough. However, once the graph is loaded, it cannot be exported back into a JSON format for LLM use.

The knowledge graph contains two types of information: data required to transform PyDough code into SQL, and context required by LLMs to generate PyDough code. For code generation, not all metadata is relevant to the LLM. Therefore, we need the ability to extract the graph back out of PyDough.

NOTE: The primary requirement is simply to be able to export the JSON metadata graph back from PyDough. Once this export capability exists, any formatting, customization, and filtering can be handled externally outside of the DSL.

Proposed Agent Tools
In addition to a full export, having this JSON foundation will allow us to build useful tools for AI agents, such as:

1. Collection List Lookup: Get a dictionary of all collections and their descriptions in the following JSON format:

{
  "collection1": "description1",
  "collection2": "description2",
  ...
}

2. Filtered Collection Lookup: Get a dictionary containing only a specific, filtered subset of collections.