Ignore Content-Type parameters when polling MSC4108 rendezvous channel

MSC4108RendezvousSession.receive() compared the Content-Type header
against "text/plain" with strict string equality. If a reverse proxy in
front of the rendezvous server appends parameters to the header (e.g.
nginx's `charset` directive rewrites it to `text/plain; charset=utf-8`),
every incoming payload was treated as "no new message" and silently
discarded, deadlocking QR code login until the session expired.

Parse out the media type per RFC 9110 (parameters follow the type/subtype
after a semicolon; type names are case-insensitive) before comparing.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QHDRWR469Sb2hQAoYRS3oN
Signed-off-by: Krithin Sitaram <1389679+krithin@users.noreply.github.com>

Author: krithin

spec/unit/rendezvous/MSC4108RendezvousSession.spec.ts

@@ -210,6 +210,34 @@ describe("MSC4108RendezvousSession", () => {
         }
     });
 
+    it("GET with content-type parameters, e.g. added by an intermediary proxy", async function () {
+        const client = makeMockClient({ userId: "@alice:example.com", deviceId: "DEVICEID", msc4108Enabled: false });
+        const transport = new MSC4108RendezvousSession({
+            client,
+            fallbackRzServer: "https://fallbackserver/rz",
+        });
+        {
+            // initial POST
+            fetchMock.postOnce("https://fallbackserver/rz", {
+                status: 201,
+                body: { url: "https://fallbackserver/rz/123" },
+            });
+            await expect(transport.send("foo=baa")).resolves.toStrictEqual(undefined);
+            await fetchMock.callHistory.flush(true);
+        }
+        {
+            // GET where the content-type has a charset parameter appended, as nginx's
+            // `charset` directive does — the payload must still be received
+            fetchMock.getOnce("https://fallbackserver/rz/123", {
+                status: 200,
+                body: "foo=baa",
+                headers: { "content-type": "text/plain; charset=utf-8", "etag": "aaa" },
+            });
+            await expect(transport.receive()).resolves.toEqual("foo=baa");
+            await fetchMock.callHistory.flush(true);
+        }
+    });
+
     it("POST and PUTs", async function () {
         const client = makeMockClient({ userId: "@alice:example.com", deviceId: "DEVICEID", msc4108Enabled: false });
         const transport = new MSC4108RendezvousSession({

src/rendezvous/transports/MSC4108RendezvousSession.ts

@@ -203,7 +203,10 @@ export class MSC4108RendezvousSession {
             // rely on server expiring the channel rather than checking ourselves
 
             const etag = poll.headers.get("etag") ?? undefined;
-            if (poll.headers.get("content-type") !== "text/plain") {
+            // Strip any parameters from the content type, e.g. a `charset` appended
+            // by an intermediary proxy, as only the media type itself matters.
+            const contentType = poll.headers.get("content-type")?.split(";", 1)[0].trim().toLowerCase();
+            if (contentType !== "text/plain") {
                 this.etag = etag;
             } else if (poll.status === 200) {
                 if (!etag) {