Resources: subscriptions/listen model on modern servers (era fork) (#1630) (#1730)

Fork resource subscriptions by protocol era: legacy keeps resources/subscribe; modern (2026-07-28) models subscriptions as a filter over one long-lived subscriptions/listen stream with reconnect-by-re-listen (capped backoff), an optimistic Connecting→Listening header badge, and a fix so the browser proxy no longer blocks listen() ~60s. Closes #1630.

test-servers/configs/subscriptions-legacy-http.json

@@ -0,0 +1,14 @@
+{
+  "serverInfo": {
+    "name": "subscriptions-legacy",
+    "version": "1.0.0"
+  },
+  "tools": [{ "preset": "update_resource" }],
+  "resources": [{ "preset": "numbered_resources", "params": { "count": 3 } }],
+  "subscriptions": true,
+  "listChanged": { "resources": true },
+  "transport": {
+    "type": "streamable-http",
+    "port": 3221
+  }
+}

test-servers/configs/subscriptions-modern-http.json

@@ -0,0 +1,14 @@
+{
+  "serverInfo": {
+    "name": "subscriptions-modern",
+    "version": "1.0.0"
+  },
+  "resources": [{ "preset": "numbered_resources", "params": { "count": 3 } }],
+  "subscriptions": true,
+  "listChanged": { "resources": true },
+  "transport": {
+    "type": "streamable-http",
+    "port": 3220,
+    "modern": true
+  }
+}