Show specific error messages on team unlinking failure (follow-up to #6953)

[suzit-10]

What type of PR is this? (check all applicable)

• 🍕 Feature
• 🐛 Bug Fix
• 📝 Documentation
• 🧑‍💻 Refactor
• ✅ Test
• 🤖 Build or CI
• ❓ Other (please specify)

Describe this PR

This PR enhances the existing team unlinking feature by displaying contextual error messages based on the failure reason (e.g. team assigned as validation/mapping/manager permission).

The initial unlinking functionality was introduced in #6953. This follow-up improves user feedback by interpreting the backend Subcode in unlink failure responses and showing relevant messages.

Changes

• Display specific error messages depending on the Subcode returned in unlinking failure responses.
• Improved error handling flow in the unlinking logic for better clarity.
• Minor code clean-ups for readability.

Checklist before requesting a review

• 📖 Read the Tasking Manager Contributing Guide: https://github.com/hotosm/tasking-manager/blob/develop/docs/developers/contributing.md
• 📖 Read the HOT Code of Conduct: https://docs.hotosm.org/code-of-conduct
• 👷‍♀️ Create small PRs. In most cases, this will be possible.
• ✅ Provide tests for your changes.
• 📝 Use descriptive commit messages.
• 📗 Update any related documentation and include any relevant screenshots.
• 🔠 Does this PR introduce or change any environment variables? If so, make sure to specify this change in the description.

[sonarqubecloud]

Quality Gate passed

Issues
1 New issue
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud