add shared chat session contract

contracts/v1/shared-chat-session-response.json

@@ -0,0 +1,23 @@
+{
+  "$schema": "https://json-schema.org/draft/2020-12/schema",
+  "$id": "https://trixie.reaperofveriod.nl/contracts/v1/shared-chat-session-response.json",
+  "title": "Twitch Shared Chat session response",
+  "type": ["object", "null"],
+  "additionalProperties": false,
+  "required": ["session_id", "host_broadcaster_id", "participants", "created_at", "updated_at"],
+  "properties": {
+    "session_id": { "type": "string", "minLength": 1 },
+    "host_broadcaster_id": { "type": "string", "minLength": 1 },
+    "participants": {
+      "type": "array",
+      "items": {
+        "type": "object",
+        "additionalProperties": false,
+        "required": ["broadcaster_id"],
+        "properties": { "broadcaster_id": { "type": "string", "minLength": 1 } }
+      }
+    },
+    "created_at": { "type": "string", "format": "date-time" },
+    "updated_at": { "type": "string", "format": "date-time" }
+  }
+}

fixtures/v1/shared-chat-session.response.json

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+{
+  "session_id": "shared-session-1",
+  "host_broadcaster_id": "1234",
+  "participants": [{ "broadcaster_id": "1234" }, { "broadcaster_id": "5678" }],
+  "created_at": "2026-01-01T00:00:00Z",
+  "updated_at": "2026-01-01T00:00:00Z"
+}