test: add http2 handler e2e suite and tighten host-mapper + idle-timeout tests

Add tests/e2e/http2_test.go covering the http2 handler/listener/connector/dialer
end-to-end through the canonical gost-as-client pattern (8 subtests: forward,
auth, bypass, probeResist/metadata, h2 stream multiplexing). Include
testdata/http2/{server,server_auth,server_bypass,server_proberesist,client,
client_auth}.yaml. Note that h2/h2c listeners pair with the tunnel handler
and only http2 listener pairs with handler http2 (reads r/w from metadata).

Tighten TestDNSHostMapper by pointing the handler at an unreachable upstream
so unmapped names fail at the exchanger, confirming the host-mapper path was
the sole reason mapped names resolved. Fix http_idle_timeout.py to speak HTTP
through the CONNECT tunnel rather than a raw ping, matching the echo backend.

Co-Authored-By: Claude <noreply@anthropic.com>

tests/e2e/dns_test.go

@@ -168,9 +168,12 @@ func (s *DNSSuite) TestDNSBypass() {
 // TestDNSHostMapper verifies DNS resolution via the host mapper before
 // reaching the upstream exchanger.
 //
-// Config: server_hosts.yaml maps mapped.example.com → 10.0.0.100 with no
-// working upstream. The handler checks the host mapper before the exchange
-// path, so mapped names resolve without needing any upstream DNS.
+// Config: server_hosts.yaml maps mapped.example.com → 10.0.0.100 and
+// points the handler at an unreachable upstream (udp://127.0.0.1:1).
+// The handler checks the host mapper before the exchange path, so
+// mapped names resolve without needing any upstream DNS. Unmapped
+// names must fall through to the exchanger, which fails — confirming
+// the host-mapper path was the only reason mapped names worked.
 func (s *DNSSuite) TestDNSHostMapper() {
 	gostC, err := RunGostContainerWithFiles(s.ctx, SharedNetworkName,
 		"testdata/dns/server_hosts.yaml",