[release-branch.go1.25] html/template: properly escape URLs in meta content attributes

The meta tag can include a content attribute that contains URLs, which
we currently don't escape if they are inserted via a template action.
This can plausibly lead to XSS vulnerabilities if untrusted data is
inserted there, the http-equiv attribute is set to "refresh", and the
content attribute contains an action like `url={{.}}`.

Track whether we are inside of a meta element, if we are inside of a
content attribute, _and_ if the content attribute contains "url=". If
all of those are true, then we will apply the same URL escaping that we
use elsewhere.

Also add a new GODEBUG, htmlmetacontenturlescape, to allow disabling this
escaping for cases where this behavior is considered safe. The behavior
can be disabled by setting htmlmetacontenturlescape=0.

Updates #77954
Fixes #77971
Fixes CVE-2026-27142

Change-Id: I9bbca263be9894688e6ef1e9a8f8d2f4304f5873
Reviewed-on: https://go-internal-review.googlesource.com/c/go/+/3360
Reviewed-by: Neal Patel <nealpatel@google.com>
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-on: https://go-internal-review.googlesource.com/c/go/+/3644
Reviewed-by: Damien Neil <dneil@google.com>
Commit-Queue: Roland Shoemaker <bracewell@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/752101
Auto-Submit: Gopher Robot <gobot@golang.org>
TryBot-Bypass: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>