fix(nango-yaml): fine tune parsing

[bodinsamuel]

Describe your changes

Fix more edge cases and fine tune messaging

• Handle union type in input/output correctly
• Fix: Action template without an input
• Be more explicit about unsupported typescript syntax
• Do not complain about anonymous model in case of union, it's a legitimate use case of anonymous model
• Do not complain about action input model reuse, it's too noisy to be useful

[khaliqgant]

Adjustment for clarity in the error response requested

[bodinsamuel]

Thanks @khaliqgant 👌🏻