feat: add Labkit::Context.clear for resetting context between tests

Description

Adds Labkit::Context.clear to reset the context stack stored in Thread.current[:labkit_contexts]. This is useful in test environments where contexts are pushed outside of with_context and can otherwise leak state into subsequent tests. It also provides a clean reset point for thread-reusing frameworks (e.g. Puma, Sidekiq) that need to clear context at the start of each request or job.

Without this method, callers would need to reference the internal :labkit_contexts thread-local key directly to achieve the same effect.

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