feat: implement rules customization unit tests using feature files

[sinapah]

Issue

Implements the unit tests in #205 using feature files.

Solution

• Dependencies: Added pytest-bdd to dev dependencies to enable Gherkin feature files as the test driver.

• Feature Files: Created tests/features/alert_rule_customization.feature containing 20 behavioral scenarios:

• Removal target rules: By alert name, group name, label, annotation, and topology label.

• Patch updates:** For duration, alert name, expr, labels, and annotations.

• Rule management: Adding rules under the fixed key.

• Core properties: Input immutability, operation ordering, and instance reusability.

• BDD Tests: Added tests/test_rules_customization.py to bind pytest-bdd step definitions to the feature file, replacing the legacy all-in-one test module.

• Schema Tests: Added tests/test_rules_customization_schema.py (extracted from the old module) to isolate schema validation and no-op config tests, keeping complex bad-input matrices in clean unittest cases.

• Shared Fixtures: Added tests/conftest.py containing the sample_alerts fixture and find_rule helper function for use across test suites.

Checklist

• I have added or updated relevant documentation.
• PR title makes an appropriate release note and follows conventional commits syntax.
• Merge target is the correct branch, and relevant tandem backport PRs opened.
• This change warrants a release, so I have updated the project.version field in the pyproject.toml file.

Context

Testing Instructions

Upgrade Notes

[sed-i]

Should remove, patch be covered in separate feature files?

[sinapah]

Yes. Not just that, but I think now that we've decided to do away with add, we can have three feature files: one for each of patch, remove, and remove+patch`.

[sed-i]

• This seems to suggest that the feature file isn't self-contained - there's a implicit dependency on some python code to make "LowThroughput" available. Not sure, but seems like an antipattern. Should we try to make the feature file more self contained? E.g. using the docstring fixture.
• I think there is room for making greater use of parsers.parse().

[sinapah]

1. You're right in that the feature file isn't fully self-contained. However, this seems like (more or less) normal BDD behaviour? In the case of alerts, passing them as docstrings to the set up functions would make the feature file very verbose and it would considerably decrease readability.
2. Yes, let's use parsers.parse().

[sed-i]

Why not inline this (and others) in the feature file's docstring?
https://pytest-bdd.readthedocs.io/en/latest/#scenario-outlines

[sinapah]

This was the original unit testing added. Since we've revamped the testing to rely on Gherkin, we can scrap it.

[sinapah]

Use a target fixture, look at https://pytest-bdd.readthedocs.io/en/latest/