refactor: GenericUpsellModal to use GenericModal

[aleksandernsilva]

Proposed changes (including videos or screenshots)

This PR refactors GenericUpsellModal to use GenericModal internally, reducing code duplication and making future maintenance easier.

It also improves the Storybook stories and adds unit tests to ensure component reliability.

Issue(s)

CTZ-199

Steps to test or reproduce

Further comments

[dionisio-bot]

Looks like this PR is not ready to merge, because of the following issues:

• This PR is targeting the wrong base branch. It should target 7.9.0, but it targets 7.8.0

Please fix the issues and try again

If you have any trouble, please check the PR guidelines

[changeset-bot]

⚠️ No Changeset found

Latest commit: 8dcbea0

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[github-actions]

PR Preview Action v1.6.1
🚀 View preview at https://RocketChat.github.io/Rocket.Chat/pr-preview/pr-36347/
Built to branch gh-pages at 2025-07-02 22:17 UTC. Preview will be ready when the GitHub Pages deployment is complete.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 64.92%. Comparing base (fd478a7) to head (8dcbea0).
Report is 1 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #36347      +/-   ##
===========================================
+ Coverage    64.88%   64.92%   +0.04%     
===========================================
  Files         3154     3156       +2     
  Lines       104741   104824      +83     
  Branches     19935    19925      -10     
===========================================
+ Hits         67957    68061     +104     
+ Misses       34099    34081      -18     
+ Partials      2685     2682       -3     

Flag	Coverage Δ
e2e	57.68% <0.00%> (+0.01%)	⬆️
unit	69.75% <100.00%> (+0.05%)	⬆️

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[dougfabris]

Looks great to me!