cuda.core: allow updating Buffer deallocation streams - #2602

Make the deallocation stream's context current around free/unmap/MR cleanup so destruction no longer depends on ambient CUDA context, and wire cuCtxSetCurrent into the resource-handles driver table.

Add keyword-only stream= on Buffer/ManagedBuffer.from_handle when mr owns the pointer, bind it at construction, and cover teardown with no or foreign current context.

Stop treating CUDA_ERROR_INVALID_CONTEXT as a successful pool free, and let
explicit mr.deallocate() raise; destruction still contains errors in the
callback. Document PTDS deallocation ordering on the stream parameters and
note the context-safe Buffer teardown fix in the 1.2.0 release notes.

Require default deallocation streams to bind a current context at creation so teardown never relies on an ambiguous ambient token. Expand coverage and documentation for context-independent cleanup and failure reporting.

…ream

Ensure spawned children can bind the imported buffer's default deallocation stream before their process target starts.

…ream