Confirm that site is indeed not a WordPress site after XML-RPC validation fails

[staskus]

More context: wordpress-mobile/WordPress-iOS#18497 (comment)
Related PR wordpress-mobile/WordPress-iOS#21904

In rare cases, XML-RPC validation can fail and indicate that the site is not a WordPress site when it actually is.

To avoid this corner case, I added an additional isWPSite check after guessXMLRPCURL and before showing errors to the users:

• If WordPressOrgXMLRPCValidator determines that it's not a valid WordPress site and isWPSite (connect/site-info) API call confirms it, then we inform the user that the site is invalid
• If WordPressOrgXMLRPCValidator determines that it's not a valid WordPress site but isWPSite (connect/site-info) API suggests otherwise, we inform users about XML-RPC issues which they can then look for further assistance

Implementation details

I didn't want to change this fragile code before writing tests to it and to write tests I needed to split business logic from UI. I recommend reviewing commit by commit:

1. Extract error handling code - 9c14098
2. Move it to ViewModel - b62db73
3. Add unit tests - 76925ba
4. Add isWPSite check - a27ac06

• I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.

[staskus]

@guarani the code can be reviewed but ideally, I would do some more manual testing myself, so I'm not targeting 23.6 for sure. If you'll have time to review the code or critique the solution overall - great, I'll finish after I come back. Thanks! 🙇

[SiobhyB]

@staskus, thank you for working on this! 🙇‍♀️ I verified that the misleading "not a WordPress site" error is resolved with these changes applied:

Before	After

In addition, I went through the steps at wordpress-mobile/WordPress-iOS#17724 and tried a few ways to see if I could break login, but couldn't.

The only note I have is that, although less confusing than before, I feel some users won't know what next steps to take or perhaps even what XML-RPC is. How would you feel about adding a sentence like Please check with your hosting provider to find out if they are blocking XML-RPC access? Or perhaps even linking to our support doc? I understand that this might be out of the scope of this PR though. Let me know if I should open a separate issue regarding the wording!

I went through each commit as well as your descriptions, and the logic seems sound to me. That said, I'm not an iOS developer, so it might be wise to have @guarani or another iOS developer do another pass.

Approving based on my testing and understanding of the logic. :)

[guarani]

Approved via wordpress-mobile/WordPress-iOS#17724.

The code change looks good and I ran this repos tests locally.