Implement quiz resources selection switching mode

[AlexVelezLl]

Summary

• Implement quiz resources selection switching mode logic
• Add button to change the selection mode within the exercise preview
• Fixes missing coreString value in ResourcePreview

Compartir.pantalla.-.2025-02-26.15_59_37.mp4

References

Closes #13015.

Reviewer guidance

• With random selection mode, select resources to add to a quiz, then change the selection mode to manual
• With manual selection mode, select questions to add to a quiz, then change the selection mode to quiz
• Play combining different possibilities and using the questions settings page and the selection mode checkbox in the exercise preview.

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.18.0a1.dev0_git.78.g07639511.pex
Windows Installer (EXE)	kolibri-0.18.0a1.dev0+git.78.g07639511-windows-setup-unsigned.exe
Debian Package	kolibri_0.18.0a1.dev0+git.78.g07639511-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.18.0a1.dev0+git.78.g07639511.dmg
Android Package (APK)	kolibri-0.18.0a1.dev0+git.78.g07639511-0.1.4-debug.apk
Raspberry Pi Image	kolibri-pi-image-0.18.0a1.dev0+git.78.g07639511.zip
TAR file	kolibri-0.18.0a1.dev0+git.78.g07639511.tar.gz
WHL file	kolibri-0.18.0a1.dev0+git.78.g07639511-py2.py3-none-any.whl

[marcellamaki]

This looks good, @AlexVelezLl - it looks like it aligns with the figma well, and the code seems fine to me. I think the best way to really get a feel for this (and if there are any edge cases that we haven't thought of) will be for it to be merged and tried out in the alpha release. Thanks for the quick work here :)