[Tech Debt] Improve unit testing and internal documentation on Contexts and providers

[MichelleArk]

The various jinja contexts are fairly well documented for users, but are very unstructured objects internally. Let's take a pass at ensuring our tests provide coverage to the documentation, improving our internal documentation around the Context and Provider design, and consider any refactors along the way.

Testing

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1. unit test on-run-end context #10954
   +0
2. unit test dbt_project.yml context #10951
   +0
3. unit test properties.yml context #10953
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4. unit test profiles.yml context #10952
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5. unit test BaseContext methods / types in addition to existence of values

Documentation

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1. Internal guides on current Provider, Context and generate_ methods

Refactors

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1. Consider removing Providers in favor of parse-time and execution-time contexts.

[devmessias]

I have submitted some PRs that fix issues in the unit tests, along with one suggestion. I was also working on resolving the naming collision problem when using sources. However, I’d like to wait for decisions on my previous issues and PRs before making a new PR. Would it be a good idea to put here these issues as technical debt to be addressed/reviewed?