Fix output routing when threads inherit context

[emecii]

Closes #1491

A background thread can inherit the executing cell's ContextVar state (the default on free-threaded CPython, and also possible with copy_context). Because ipykernel treated any context-local parent as belonging to the current thread, a persistent worker stayed pinned to the cell that created it.

Store the setting thread ID with each context-local parent, and fall back to the current global parent when another thread observes an inherited context. Explicit set_parent and set_thread_parent calls in the worker still pin output as before.

The regression runs a worker in a copied context and covers both stream and display output across three cells, including explicit global and temporary thread-local overrides.

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