Don't send absolute paths when POSTing the Certificate.upload_cert() request

[ddribin]

Send hardcoded names ("server.key", "server.crt", and "ca.crt") as the filename of each multipart part.

Resolves #470

🗃️ Summary

What does this pull request do?

Send hardcoded names ("server.key", "server.crt", and "ca.crt") as the filename of each multipart part.

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🚀 Motivation & Problem Statement

Why is this PR needed?

Resolves #470

🔧 Implementation Details

What changes were made?

• List the files affected and the nature of the changes (add, modify, delete).
• Mention any new tests added or existing ones updated.
• If it touches the documentation, indicate which sections were updated.

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🏁 Checklist

• I have read and followed the Contributing guidelines.
• All new or modified code is covered by unit tests (tests/).
• Tests pass locally (pytest).
• I added or updated documentation where necessary.
• I updated the changelog or added a new section if this is a major change.
• I followed the style guidelines (black, flake8, etc.).
• I ran pre-commit and addressed any linting issues.

Note: If you added a new API wrapper, please update docs/ and add the
corresponding entries to APIs - Supported APIs in the README.

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📝 Related Issue

Issue number(s) this PR addresses
#123

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🔨 Additional Notes

Anything else that reviewers should know?

• Known limitations
• Required manual steps
• Performance implications

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😎 Test & Build Status

Run the following locally before opening the PR:

pip install -r requirements-dev.txt
pre-commit run --all-files
pytest

pre-commit isn't installed from requirements-dev.txt, and pytest was failing with what looks like an unrelated error.

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👀 Screenshots / Media

If your changes include UI changes, API responses, or other visual elements,
please attach screenshots or GIFs.

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Thank you for contributing! 🙏