fix(tlon): add timeout to SSE client fetch calls (CWE-400) (#5926)

Add timeout protection to prevent indefinite hangs when Urbit server
becomes unresponsive or network partition occurs.

Changes:
- Add AbortSignal.timeout(30_000) to 7 one-shot fetch calls
- Add AbortController with 60s connection timeout to SSE stream fetch
  (clears timeout after headers received to avoid aborting active stream)

Affected methods: sendSubscription, connect, openStream, poke, scry, close

Fixes #5266

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

extensions/tlon/src/urbit/sse-client.ts

@@ -114,6 +114,7 @@ export class UrbitSSEClient {
         Cookie: this.cookie,
       },
       body: JSON.stringify([subscription]),
+      signal: AbortSignal.timeout(30_000),
     });
 
     if (!response.ok && response.status !== 204) {
@@ -130,6 +131,7 @@ export class UrbitSSEClient {
         Cookie: this.cookie,
       },
       body: JSON.stringify(this.subscriptions),
+      signal: AbortSignal.timeout(30_000),
     });
 
     if (!createResp.ok && createResp.status !== 204) {
@@ -152,6 +154,7 @@ export class UrbitSSEClient {
           json: "Opening API channel",
         },
       ]),
+      signal: AbortSignal.timeout(30_000),
     });
 
     if (!pokeResp.ok && pokeResp.status !== 204) {
@@ -164,14 +167,23 @@ export class UrbitSSEClient {
   }
 
   async openStream() {
+    // Use AbortController with manual timeout so we only abort during initial connection,
+    // not after the SSE stream is established and actively streaming.
+    const controller = new AbortController();
+    const timeoutId = setTimeout(() => controller.abort(), 60_000);
+
     const response = await fetch(this.channelUrl, {
       method: "GET",
       headers: {
         Accept: "text/event-stream",
         Cookie: this.cookie,
       },
+      signal: controller.signal,
     });
 
+    // Clear timeout once connection established (headers received)
+    clearTimeout(timeoutId);
+
     if (!response.ok) {
       throw new Error(`Stream connection failed: ${response.status}`);
     }
@@ -279,6 +291,7 @@ export class UrbitSSEClient {
         Cookie: this.cookie,
       },
       body: JSON.stringify([pokeData]),
+      signal: AbortSignal.timeout(30_000),
     });
 
     if (!response.ok && response.status !== 204) {
@@ -296,6 +309,7 @@ export class UrbitSSEClient {
       headers: {
         Cookie: this.cookie,
       },
+      signal: AbortSignal.timeout(30_000),
     });
 
     if (!response.ok) {
@@ -364,6 +378,7 @@ export class UrbitSSEClient {
           Cookie: this.cookie,
         },
         body: JSON.stringify(unsubscribes),
+        signal: AbortSignal.timeout(30_000),
       });
 
       await fetch(this.channelUrl, {
@@ -371,6 +386,7 @@ export class UrbitSSEClient {
         headers: {
           Cookie: this.cookie,
         },
+        signal: AbortSignal.timeout(30_000),
       });
     } catch (error) {
       this.logger.error?.(`Error closing channel: ${String(error)}`);