Don't throw InvalidOperationException in API functions/attributes if Allure is not running

[delatrie]

Context

Currently, if Allure is not running, attempting to call functions from AllureApi or ExtendedApi or a function marked with [AllureStep], [AllureBefore], or [AllureAfter] leads to InvalidOperationException with a somewhat cryptic message about Allure context. This makes it challenging to selectively disable Allure in some environments (e.g., on a developer's machine) while enabling it in others (like a CI workflow).

This PR makes all those functions and attributes usable in such scenarios. Now, they check the Allure context first. If it doesn't allow the function to do its job, the function becomes a noop:

• Functions that add/set metadata do nothing and exit (SetTestName, AddLabel, etc).
• Lambda step/fixture functions call the functions they wrap and return the result. They don't do anything else.
• Step/fixture attributes also call their target functions directly and do nothing more. In particular, they don't attempt to format the arguments, which saves performance.

Closes #445

Checklist

• Sign Allure CLA
• Provide unit tests