test: SOCKS5 proxy: expect that connection may be reset when forwarding #35510

The forward_sockets() function used by the SOCKS5 proxy forwards data between two connected sockets. It might happen that one of those sockets gets closed/reset abruptly, without sending EOF first. This is to be expected if e.g. bitcoind is shutdown and shouldn't result in noisy harmless messages like:

2026-06-03T13:23:56.966859Z TestFramework.socks5 (ERROR): socks5 request handling failed (running True)
Traceback (most recent call last):
  File ".../socks5.py", line 199, in handle
    forward_sockets(self.conn, conn_to, self.wakeup_socket_pair[1], self.serv)
    ~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../socks5.py", line 76, in forward_sockets
    data = s.recv(4096)
ConnectionResetError: [Errno 104] Connection reset by peer

Instead turn this into a debug log message with a nice prefix containing enough information to identify the two forwarded sockets.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/35510.

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Concept ACK! I was about to open PR myself, haha :)

I wonder if it's worth it to also catch BrokenPipeError, although IIUC it's harder to hit. My attempt at this was: https://github.com/danielabrozzoni/bitcoin/commit/d9ccf9c4ec5616d5f1dabcaf2ab7bb4c73a8fdbc

Also, I was wondering if this could be removed in p2p_private_broadcast.py - was it added because of the ConnectionResetError, or for some other reason? https://github.com/bitcoin/bitcoin/blob/46d0e21d758c598017b5a9d86381c1c8ccfbe94d/test/functional/p2p_private_broadcast.py#L382-L385

@optout21, you did some improvements around this code recently, might be interested into this PR @danielabrozzoni, you reported this in #35410#pullrequestreview-4419151702, might be interested as well

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🚧 At least one of the CI tasks failed. _{Task lint: https://github.com/bitcoin/bitcoin/actions/runs/27337603147/job/80765794480} _{LLM reason (✨ experimental): CI failed due to a Python linter syntax error (invalid f-string in test/functional/test_framework/socks5.py:91, “expecting '}'”).}

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