Fix dex_tests using incorrect types and field mappings

[imdawon]

The initial release to support the dex_tests feature (https://github.com/cloudflare/cloudflare-go/pull/1209/files) contained managed network types and json struct mappings, which would cause missing test ids in the device dex test struct and likely 500 errors.

This PR fixes the code which would have caused those issues.

Has your change been tested?

A test, which was not in the initial release, was added for the update function. It fails with the previous incorrect types and mappings, and passes with the updated code.

Types of changes

What sort of change does your code introduce/modify?

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.
• This change is using publicly documented (api.cloudflare.com or developers.cloudflare.com) and stable APIs.

[github-actions]

changelog detected ✅

[github-actions]

This functionality has been released in v0.62.0.

For further feature requests or bug reports with this functionality, please create a new GitHub issue following the template. Thank you!