Improve error messages when a pending Trusted Publisher's project name already exists

[facutuesca]

The problem

There are some cases where we validate the name of a new project, where the error messages could be better:

• When creating a pending Trusted Publisher with a project name that already exists, the error message includes a link to the project's settings. This link is only useful if the user is the owner of the existing project, otherwise it's confusing (see Trusted Publisher error message is confusing when project exists but user doesn't have ownership #17392).

The implementation

This PR removes the enum used for describing the invalid project name errors, replacing it with simple classes that allow including some metadata in each error object. This metadata allows us to tell if the current user is the owner of the existing project, which we then use to display better error messages.

# Before
class ProjectNameUnavailableReason(enum.Enum):
    Invalid = "invalid"
    Stdlib = "stdlib"
    AlreadyExists = "already-exists"
    Prohibited = "prohibited"
    TooSimilar = "too-similar"

# After
class ProjectNameUnavailableInvalid:
    pass
class ProjectNameUnavailableStdlib:
    pass
class ProjectNameUnavailableProhibited:
    pass

class ProjectNameUnavailableExisting:
    def __init__(self, existing_project: Project):
        self.existing_project: Project = existing_project
class ProjectNameUnavailableSimilar:
    def __init__(self, similar_project: Project):
        self.similar_project: Project = similar_project

ProjectNameUnavailableError = (
    ProjectNameUnavailableInvalid
    | ProjectNameUnavailableStdlib
    | ProjectNameUnavailableExisting
    | ProjectNameUnavailableProhibited
    | ProjectNameUnavailableSimilar
)

Screenshots

Creating a pending Trusted Publisher

Project already exists and user is owner

Project already exists and user is not owner

---

This fixes #17392

cc @di @woodruffw

[samuelcolvin]

Thanks so much for fixing so quickly ❤️ .

[miketheman]

Coming along nicely, I think we're almost there! A few last bits to consider, and then I think we're ready to go. Let me know when it's ready for re-review by clicking the 🔄 button on my username.

[miketheman]

Well done!

[ewdurbin]

https://python-software-foundation.sentry.io/share/issue/27a8e5055a0c4c54a2fcceb2c3479241/