http2: enable HTTP/2 in wrap configureTransport

The go1.27 wrap implementation of ConfigureTransport is a no-op, and
ConfigureTransports only registers the protocol with net/http; neither
enables HTTP/2 on the transport or initializes TLSClientConfig. The
pre-wrapping transport.go did both.

As a result, callers that enable HTTP/2 on a transport with a custom
TLSClientConfig or dialer (where net/http does not enable HTTP/2
automatically) send requests over HTTP/1 to an HTTP/2 server. Without
"h2" in NextProtos this is a silent downgrade to HTTP/1.1; with it, the
connection negotiates h2 at the TLS layer but is read by the HTTP/1
transport, failing with "net/http: HTTP/1.x transport connection broken:
malformed HTTP response".

This is the client-side counterpart of the server-side fix in CL 782940.
configureTransport now delegates to configureTransports, which ensures a
non-nil TLSClientConfig and enables net/http's native HTTP/2 via
Transport.Protocols.

TestConfigureTransport is gated to the legacy build
(!(go1.27 && !http2legacy)) and does not compile into the wrapper build,
which is why this regressed silently. The new transport_configure_test.go
is untagged and exercises both ConfigureTransport and ConfigureTransports
under the legacy and wrap builds.

Fixes golang/go#79778

Change-Id: Ibc27bced8679fcb3baa7953bad12315da07535fb
Reviewed-on: https://go-review.googlesource.com/c/net/+/785900
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Nicholas Husin <nsh@golang.org>