Fix an issue where the All Domains flow wasn't taking into account sites with existing plan

[salimbraksa]

Fixes #22268

Description

This PR resolves an issue where users purchasing a domain, linked to a site that already has a plan, through the All Domains flow get stuck in the Plans flow.

Test Instructions

N.B: Sandboxing is required to test domain checkout

Case 1: Site with Existing Plan

1. Log into the site that already has a plan
2. Navigate to Me > All Domains
3. Tap "+"
4. Select a domain
5. Tap "Choose Site" button
6. Expect the Checkout web view to appear
7. Purchase the domain to verify the checkout flow works.

Case 2: Site without plan ( Regression )

1. Log into the site that doesn't have a plan
2. Navigate to Me > All Domains
3. Tap "+"
4. Select a domain
5. Tap "Get Domain" button
6. Expect the Plans web view to appear
7. Select a plan, go verify the checkout flow works as expected.

Case 3: Domain with No-Site ( Regression )

1. Log into the site that already has a plan
2. Navigate to Me > All Domains
3. Tap "+"
4. Select a domain
5. Tap "Get Domain" button
6. Expect the Checkout web view to appear
7. Purchase the domain to verify the checkout flow works.

Next

• Address the // TODO: I added in RegisterDomainCoordinator.swift.
• Add Unit Tests for RegisterDomainCoordinator.

Regression Notes

1. Potential unintended areas of impact Purchasing a domain through the All Domains flow.

2. What I did to test those areas of impact (or what existing automated tests I relied on) Manual testing.

3. What automated tests I added (or what prevented me from doing so) None.

PR submission checklist:

• [x] I have completed the Regression Notes.
• [x] I have considered adding unit tests for my changes.
• [x] I have considered adding accessibility improvements for my changes.
• [x] I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

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📲 You can test the changes from this Pull Request in WordPress Alpha by scanning the QR code below to install the corresponding build.

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	Configuration	Release-Alpha
	Build Number	pr22343-b8b430f
	Version	24.0
	Bundle ID	org.wordpress.alpha
	Commit	b8b430f2aaef115087adab5accb3815f911487e2
	App Center Build	WPiOS - One-Offs #8551

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📲 You can test the changes from this Pull Request in Jetpack Alpha by scanning the QR code below to install the corresponding build.

	App Name	Jetpack Alpha
	Configuration	Release-Alpha
	Build Number	pr22343-b8b430f
	Version	24.0
	Bundle ID	com.jetpack.alpha
	Commit	b8b430f2aaef115087adab5accb3815f911487e2
	App Center Build	jetpack-installable-builds #7578

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[hassaanelgarem]

@salimbraksa Can you briefly explain how these changes fixed the issue? 🙏

[salimbraksa]

Following Povilas' comment, it appears these changes don't resolve the issue. I'm converting this PR to a Draft and deprioritizing it to focus on the In-App Feedback Prompt UI instead.