fix: Synchronous Pipeline Callbacks

[monoxgas]

Notes

• Removing explicit checks for async then/map/watch callbacks

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Generated Summary

• Removed explicit async function checks in callback iterations for the "map", "then", and ChatPipeline methods, simplifying callback registration.
• Updated type annotations in protocol definitions (_ThenChatCallback and _MapChatCallback) to accept both Awaitable and direct returned values.
• Adjusted _wrap_watch_callback to capture the callback result and conditionally await it if necessary.
• Modified test for message slicing to assert that an "Empty string target provided" warning is raised when slicing with an empty string.

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