Relax Kotlin contracts for Executable passed to assertTimeout

[marcphilipp]

Overview

Since Executable is allowed/expected to throw exceptions, the
semantics of being called EXACTLY_ONCE are blurry because it might not
be executed entirely when throwing an exception. Since the Kotlin 2.1
compiler emits a warning for these parameters, we're relaxing their
contracts to be called AT_MOST_ONCE instead.

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Definition of Done

• There are no TODOs left in the code
• Method preconditions are checked and documented in the method's Javadoc
• Coding conventions (e.g. for logging) have been followed
• Change is covered by automated tests including corner cases, errors, and exception handling
• Public API has Javadoc and @API annotations
• Change is documented in the User Guide and Release Notes