refactor: align test_agent_rbac with testing guide

[sujaan-iqbal]

Purpose / Description

Refactor tests/test_agent_rbac.py to better align with the Testing Guide by improving test naming, readability, and consistency without changing behavior

Fixes

• Fixes Refactor existing test files to follow the Testing Guide #1435

Approach

Renamed tests to use more behavior-focused names.
Added explicit return type annotations (-> None) to test methods.
Applied a consistent Arrange / Act / Assert structure.
Improved readability while preserving existing test coverage and behavior.

How Has This Been Tested?

Ran the targeted test file locally:

cd observal-server
python -m uv run pytest ../tests/test_agent_rbac.py -q

Output:
5 passed in 11.44s

Learning (optional, can help others)

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Checklist

Please, go through these checks before submitting the PR.

• You have a descriptive commit message with a short title (first line, max 50 chars).
• You have commented your code, particularly in hard-to-understand areas
• You have performed a self-review of your own code
• UI changes: include screenshots of all affected screens (in particular showing any new or changed strings)

AI Assistance

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• Was the generated code manually reviewed and tested?

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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