fix: Update the severity param type for zabbix provider's change_severity method

[dvora-ns]

Closes #5266

📑 Description

The backend logs the following error while processing the request for /providers endpoint.

keep.providers.providers_factory: Could not get provider zabbix_provider methods. (2 validation errors for ProviderMethodParam
expected_values -> 0
str type expected (type=type_error.str)
expected_values -> 1
str type expected (type=type_error.str))

This occurs because change_severity method has a parameter of type Union[int, str] that the provider factory is not designed to handle. Hence, I have updated the parameter type to string and added validations to make sure the provided value is valid.

✅ Checks

• My pull request adheres to the code style of this project
• My code requires changes to the documentation
• I have updated the documentation as required
• All the tests have passed

ℹ Additional Information

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[shahargl]

Lgtm

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1 Skipped Deployment

Project	Deployment	Preview	Comments	Updated (UTC)
keep	Ignored	Preview		Aug 23, 2025 9:04pm

[github-actions]

🎉 You're on fire, @dvora-ns! Three PRs merged and counting! 🔥🎉

Your consistent contributions are truly impressive. You're becoming a valued member of our community! 💖
Have you considered taking on some more challenging issues? We'd love to see what you can do! 💪

Remember, the team is always here to support you. Keep blazing that trail! 🚀