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

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.

```[tasklist]
### Testing
- [ ] https://github.com/dbt-labs/dbt-core/issues/10954
- [ ] https://github.com/dbt-labs/dbt-core/issues/10951
- [ ] https://github.com/dbt-labs/dbt-core/issues/10953
- [ ] https://github.com/dbt-labs/dbt-core/issues/10952
- [ ] unit test BaseContext methods / types in addition to existence of values
```

```[tasklist]
### Documentation
- [ ] Internal guides on current `Provider`, `Context` and `generate_` methods
```

```[tasklist]
### Refactors
- [ ] Consider removing `Providers` in favor of parse-time and execution-time contexts.
```