[Issue] Reduce TierPriceManagement class complexity by quick refactor

[m2-assistant]

This issue is automatically created based on existing pull request: #29202: Reduce TierPriceManagement class complexity by quick refactor

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Preconditions (*)

I had to debug the issue of removing Tier price. I found the class complex and code style rather poor.
That is why I extracted some repeated code to separate private methods with singular responsibility and explicit types.

Related Pull Requests

Fixed Issues (if relevant)

1. Fixes magento/magento2#<issue_number>

Steps to reproduce (*)

1. ...
2. ...

Actual Result:

Expected Result:

Questions or comments

Contribution checklist (*)

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds are green)