[Shipping Labels Revamp] Add Improvements and bug fixes to Packages UI

[ThomazFB]

Summary

Add some adjustments to the Package UI selection:

• Introduce a working Selector for the Package Type
• Add index resilience to the Saved and Carrier package list to avoid race conditions causing Index errors
• Introduce Fragment Result handling to allow the main Shipping Labels form to receive the selected Package Data
• Unify the ViewModel Package selection event system to a simplified one for better control over the Package data flow

How to Test

1. As the Package selection system is not yet integrated with real endpoints, it's not possible to effectively test the features introduced here. The unit tests are the only way to verify the behaviors here, which is why this work is still behind a feature flag.

Update release notes:

• [x] I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

Reviewer (or Author, in the case of optional code reviews):

Please make sure these conditions are met before approving the PR, or request changes if the PR needs improvement:

• [x] The PR is small and has a clear, single focus, or a valid explanation is provided in the description. If needed, please request to split it into smaller PRs.
• [x] Ensure Adequate Unit Test Coverage: The changes are reasonably covered by unit tests or an explanation is provided in the PR description.
• [x] Manual Testing: The author listed all the tests they ran, including smoke tests when needed (e.g., for refactorings). The reviewer confirmed that the PR works as expected on big (tablet) and small (phone) in case of UI changes, and no regressions are added.

[wpmobilebot]

📲 You can test the changes from this Pull Request in WooCommerce-Wear Android by scanning the QR code below to install the corresponding build.

	App Name	WooCommerce-Wear Android
	Platform	⌚️ Wear OS
	Flavor	Jalapeno
	Build Type	Debug
	Commit	5bdaf8b6c2f38c25828415b3453851aaa10a0b0a
	Direct Download	woocommerce-wear-prototype-build-pr12936-5bdaf8b.apk

[wpmobilebot]

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

	App Name	WooCommerce Android
	Platform	📱 Mobile
	Flavor	Jalapeno
	Build Type	Debug
	Commit	5bdaf8b6c2f38c25828415b3453851aaa10a0b0a
	Direct Download	woocommerce-prototype-build-pr12936-5bdaf8b.apk

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.33333% with 4 lines in your changes missing coverage. Please review.

Project coverage is 39.62%. Comparing base (66014e5) to head (5bdaf8b). Report is 17 commits behind head on trunk.

Files with missing lines	Patch %	Lines
...ckages/WooShippingLabelPackageCreationViewModel.kt	73.33%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## trunk #12936 +/- ## ============================================ + Coverage 39.61% 39.62% +0.01% - Complexity 5927 5928 +1 ============================================ Files 1260 1260 Lines 72832 72841 +9 Branches 9963 9963 ============================================ + Hits 28852 28863 +11 + Misses 41410 41408 -2 Partials 2570 2570 ```

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