fix(cli): Add validation rule for recursively-defined types (Issue #4135)

[Schnides123]

Adds rule and related utilities for detecting & marking cyclicly-defined types.

[dsinghvi]

It will only search within the file of the given type.

Seems like a big limitation, no?

[dsinghvi]

https://github.com/fern-api/fern/blob/main/packages/cli/yaml/validator/src/rules/no-duplicate-declarations/no-duplicate-declarations.ts#L15 Is an example of a rule that is able to check for name conflicts across files (admittedly much easier)

[Schnides123]

It will only search within the file of the given type.

Seems like a big limitation, no?

Hey, thanks for taking a look at this. I had actually ran into that while coding, but we should be covered by the existing validation rule for cyclic imports.

The only way to create a cycle across multiple files is if each file has a reference to the type defined in the other file, as seen above. That would inherently require a cyclic import, so the validation would fail it and prevent any cyclic types from being declared across files.

Since that narrows our search to only cycles defined within a single file, we can skip walking to any other files in each check and allow the rule to catch them when the main visitor for the rules reaches them normally.

As an example, we can ignore the jump from file 1 to file 2 here, allow the visitor to traverse the types in the second file, and then catch the cycle starting from type-3 or type-4.