gh-158182: http.client: don't assert on non-IPv6-zone percent-encoding in putrequest's netloc

[not-ekalabya]

• Issue: http.client: AssertionError in putrequest() on an absolute-URL netloc with percent-encoded userinfo #158182

Description

Fixes #158182.

_strip_ipv6_iface() is meant to strip an IPv6 zone id from a bracketed
literal like [fe80::1%eth0], but HTTPConnection.putrequest() passes it
the entire netloc of an absolute-URL request -- the normal shape for
HTTP-proxy requests, which is exactly what urllib.request uses when
going through a proxy. Any RFC 3986-legal percent-encoding earlier in the
netloc than an IPv6 zone id -- most plausibly percent-encoded userinfo,
which is required whenever a username or password contains @, :,
/, or % -- made the function's assert enc_name.startswith(b'[') fail,
raising a raw AssertionError instead of sending the request.

The fix only treats the input as a bracketed IPv6 literal (and strips the
zone id) when it actually starts with [; otherwise it's returned
unchanged. This also fixes the case the original assertion was meant to
guard against when both userinfo and a zone id are present (e.g.
user%41@[fe80::1%eth0]/), since the previous code partitioned on the
userinfo's % first and never reached the real zone id.

I kept this fix scoped to the crash. There's a separate, arguably more
correct improvement -- stripping userinfo from the netloc entirely before
generating the Host header, since RFC 7230 forbids userinfo in Host
regardless -- which I left out to keep this PR minimal; happy to add it if
you'd rather have both in one PR.

Found via

Found via an automated code-review pass using GLM-5.3-flash paired with
scopegrep, a semantic code-retrieval tool; independently re-verified and
fixed by hand.

Testing

Same environment limitation as gh-158181/#158183: this environment's
installed Python is 3.12, and main's stdlib can't be imported/parsed by it
directly, so I could not run Lib/test/test_httplib.py. I verified with a
standalone harness loading Lib/http/client.py via importlib:

• All three inputs that previously raised AssertionError (percent-encoded
  userinfo, percent-encoded hostname) now return the netloc unchanged.
• The existing test_ipv6host_header test's four assertions
  ([2001::]:81, [2001:102A::], [fe80::%2] -> [fe80::],
  [fe80::%2]:81 -> [fe80::]:81) were manually re-run against the fixed
  module and still pass exactly as before -- no regression to the actual
  IPv6-zone-stripping behavior this function exists for.

Please run the real test suite as part of review, since I could not do so
myself here.

Drafted with Claude Sonnet 5 (Anthropic); reviewed by @not-ekalabya before merge.

[python-cla-bot]

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[1137MOM]

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