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/http2/server_bypass.yaml

@@ -0,0 +1,20 @@
+bypasses:
+- name: bypass-0
+  matchers:
+  - 0.0.0.0/0
+
+services:
+- name: h2-proxy-bypass
+  addr: :8443
+  handler:
+    type: http2
+    auther: auther-0
+  listener:
+    type: http2
+  bypass: bypass-0
+
+authers:
+- name: auther-0
+  auths:
+  - username: user
+    password: pass