[release-branch.go1.25] net/url: reject IPv6 literal not at start of host

[release-branch.go1.25] net/url: reject IPv6 literal not at start of host

This change rejects IPv6 literals that do not appear at the start of the
host subcomponent of a URL.

For example:
  http://example.com[::1] -> rejects
  http://[::1]            -> accepts

Thanks to Masaki Hara (https://github.com/qnighy) of Wantedly.

Updates #77578
Fixes #77969
Fixes CVE-2026-25679

Change-Id: I7109031880758f7c1eb4eca513323328feace33c
Reviewed-on: https://go-internal-review.googlesource.com/c/go/+/3400
Reviewed-by: Neal Patel <nealpatel@google.com>
Reviewed-by: Roland Shoemaker <bracewell@google.com>
Reviewed-on: https://go-internal-review.googlesource.com/c/go/+/3642
Reviewed-on: https://go-review.googlesource.com/c/go/+/752100
Reviewed-by: Cherry Mui <cherryyz@google.com>
Auto-Submit: Gopher Robot <gobot@golang.org>
TryBot-Bypass: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>