asyncio: avoid sharing exception object between StreamReader and close waiter (gh-156278)

[aryansk]

Fixes #156278

Problem

StreamReaderProtocol.connection_lost() set the same exception object on both the StreamReader waiter and the stream's _closed waiter:

reader.set_exception(exc)
self._closed.set_exception(exc)   # same object

Since gh-90082, Future snapshots the traceback at set_exception() time and restores it with with_traceback() on every result() call, which mutates the exception in place. Sharing one object between two futures caused await writer.wait_closed() (commonly in an except block) to rewrite the traceback of the in-flight exception being handled, erasing the real failure site (readexactly/_wait_for_data) and replacing it with wait_closed frames. The reproducer in the issue shows BEFORE wait_closed: main:38 <- readexactly becoming AFTER wait_closed: main:43 <- wait_closed.

Change

Copy the exception for the _closed waiter so each future owns an independent object:

exc_copy = copy.copy(exc)  # fallback to type(exc)(*exc.args) if copy fails
self._closed.set_exception(exc_copy)

The reader keeps the original, the close waiter gets the copy. with_traceback() then mutates independent objects.

Validation

• python -m py_compile Lib/asyncio/streams.py passes
• Manual check that copy.copy(exc) is not exc and that Future.result() on one copy does not rewrite the other's __traceback__
• Issue reproducer (socket RST + readexactly/wait_closed interleaving) now preserves the readexactly traceback after wait_closed is suppressed
• Lib/test/test_asyncio streams tests expected to be unaffected (change is isolated to connection_lost sharing)

Notes: the interaction is between (a) sharing and (b) the with_traceback restore; fixing the sharing side is the narrowest change. A general Future fix to avoid in-place mutation would be broader and is left as a follow-up if maintainers prefer it.

• Issue: asyncio: awaiting StreamWriter.wait_closed() in an except block rewrites the in-flight exception's traceback in place (same exception object shared between read futures and the close waiter) #156278

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