fix(tests): stop vulture failing on the fake transports' send signature

[ajslater]

make lint exits 2 on develop right now, and has since
#218. bin/lint-python.sh runs
vulture . after make typecheck, and vulture flags eight
interface-mandated parameters on the two fake BaseAdapter.send
overrides I added:

tests/unit/test_rate_gate_concurrency.py:81: unused variable 'stream'
tests/unit/test_rate_gate_concurrency.py:83: unused variable 'verify'
tests/unit/test_rate_gate_concurrency.py:84: unused variable 'cert'
tests/unit/test_rate_gate_concurrency.py:85: unused variable 'proxies'
tests/util/metron_transport.py:70: unused variable 'stream'
…

My verification on #218 quoted basedpyright's "0 errors, 0 warnings"
line and grepped past the vulture step underneath it. That was my
mistake; this restores a green make lint.

Why the signature can't just drop them

• Renaming them (_stream, …) breaks at runtime.
  requests.Session.send does adapter.send(request, **kwargs)
  (sessions.py:784) with those exact keyword names, so a renamed
  parameter is a TypeError, not just a checker complaint.
• **kwargs fails the type checker. basedpyright:
  Method "send" overrides class "BaseAdapter" in an incompatible manner — Positional parameter count mismatch; base method has 7, but override has 3 (reportIncompatibleMethodOverride).

So the names stay, and the body consumes them with del plus a comment
saying why they're there.

Why not vulture_ignorelist.py

The repo already whitelists one interface-mandated parameter
(option_string, for argparse.Action.__call__), so the precedent
exists and this would be a defensible one-line alternative.

The difference is blast radius. Vulture's whitelist matches by bare
name across the whole tree — the file works by referencing a name so
it looks used. option_string is safe there because nothing else in
comicbox will ever be called that. stream, verify, cert and
proxies are ordinary words; whitelisting them would permanently blind
vulture to a genuinely dead verify or stream in production code
later, to spare two lines in two test doubles. (timeout is already
unflagged for exactly this reason — the name is used elsewhere, so
vulture assumes it's live.)

Happy to switch to the whitelist if you'd rather keep the test bodies
free of linter accommodations.

Verification

make fix && make lint && make ty && make complexity

• make lint → exit 0 (was 2)
• make ty → All checks passed!
• make complexity → exit 0, nothing over 15

make test → exit=0
2241 passed, 1 skipped, 17 warnings in 43.27s

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