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/testdata/dns/server_hosts.yaml

@@ -4,7 +4,10 @@ services:
   handler:
     type: dns
     metadata:
-      # No upstream — host mapper resolves before exchanger is ever reached.
+      # Unreachable upstream so the system-DNS fallback is bypassed.
+      # The host mapper resolves mapped names before the exchanger is
+      # reached; unmapped names must hit the exchanger and fail.
+      dns: udp://127.0.0.1:1
       timeout: 2s
   listener:
     type: dns