Fix part of #5663: Remove json lesson files

[harshsomankar123-tech]

Explanation

Fix part 02 of #5663:
This PR addresses the first half of issue #5663 by removing the JSON lesson loading path and transitioning the codebase entirely to the proto loading path (with the exception of questions). Specifically, this PR:

• Deletes the LoadLessonProtosFromAssets qualifier and removes its providers from CachingModule and CachingTestModule, alongside corresponding BUILD.bazel updates.
• Updates related domain classes (ExplorationRetrieverImpl, RevisionCardRetriever, ConceptCardRetriever, ClassroomController, TopicController, and TopicListController) by removing the constructor dependency for the flag, removing the JSON loading branches, deleting the unused *FromJson methods, and cleaning up unused org.json imports.
• Keeps the JSON loading path specifically for QuestionRetriever, as proto question loading is not yet supported.
• Removes the deprecated JSON lesson asset files from domain/src/main/assets/, keeping only questions.json.
• Cleans up the test modules that provide LoadLessonProtosFromAssets and removes those providers to ensure a successful build.

Essential Checklist

• The PR title and explanation each start with "Fix #bugnum: " (If this PR fixes part of an issue, prefix the title with "Fix part of #bugnum: ...".)
• Any changes to scripts/assets files have their rationale included in the PR explanation.
• The PR follows the style guide.
• The PR does not contain any unnecessary code changes from Android Studio (reference).
• The PR is made from a branch that's not called "develop" and is up-to-date with "develop".
• The PR is assigned to the appropriate reviewers (reference).

[harshsomankar123-tech]

@BenHenning @adhiamboperes PTAL

[BenHenning]

This cannot get merged until #6136 is first addressed. Please assign back once that's merged since it's it may affect CI.

[BenHenning]

Also @harshsomankar123-tech it seems like there are unrelated changes in this PR unless I'm mistaken.

[oppiabot]

Hi @harshsomankar123-tech, I'm going to mark this PR as stale because it hasn't had any updates for 7 days. If no further activity occurs within 7 days, it will be automatically closed so that others can take up the issue.
If you are still working on this PR, please make a follow-up commit within 3 days (and submit it for review, if applicable). Please also let us know if you are stuck so we can help you! If you're unsure how to reassign this PR to a reviewer, please make sure to review the wiki page that details the Guidance on submitting PRs.

[oppiabot]

Hi @harshsomankar123-tech, I'm going to mark this PR as stale because it hasn't had any updates for 7 days. If no further activity occurs within 7 days, it will be automatically closed so that others can take up the issue.
If you are still working on this PR, please make a follow-up commit within 3 days (and submit it for review, if applicable). Please also let us know if you are stuck so we can help you! If you're unsure how to reassign this PR to a reviewer, please make sure to review the wiki page that details the Guidance on submitting PRs.

[oppiabot]

Hi @harshsomankar123-tech, I'm going to mark this PR as stale because it hasn't had any updates for 7 days. If no further activity occurs within 7 days, it will be automatically closed so that others can take up the issue.
If you are still working on this PR, please make a follow-up commit within 3 days (and submit it for review, if applicable). Please also let us know if you are stuck so we can help you! If you're unsure how to reassign this PR to a reviewer, please make sure to review the wiki page that details the Guidance on submitting PRs.