fix: add concurrency controls for multiple sessions using the same provider (#1069)

* fix: add concurrency controls for multiple sessions using the same provider

Adds OS-inspired scheduling primitives (from HiveMind/AIMD research) to prevent
concurrent sessions from the same provider from experiencing slowdowns, random
stops, and cascading failures.

Server-side:
- Health check skips OpenCode restart when sessions are actively busy — a busy
  server under concurrent load can fail the health check timeout without being
  dead. Staleness guard forces restart if unhealthy+busy persists >2 minutes.
- Upstream SSE stall timeout scaled from 20s to 60s to avoid unnecessary
  reconnections when multiple sessions are waiting for LLM responses.

Client-side (HiveMind primitives, arXiv:2604.17111):
- Transparent retry with exponential backoff (1s→2s→4s, max 32s) for
  429/502/503/504 errors — the #1 most effective primitive from the paper.
- Circuit breaker: opens after 3 consecutive retryable errors, cooldown
  doubles each trip (30s→60s→120s, capped 128s), matching TCP AIMD.
- Per-provider session tracking with TTL eviction (1h idle sweep).
- Fetch-level retry gated on AbortError/TypeError only (not DNS failures).

Refs github-code-review skill findings (all 8 issues resolved).

* fix: use definite assignment assertion for response variable

Fixes TS2454: Variable 'response' is used before being assigned
in strict mode. The for-loop body always assigns it on every path
that reaches the post-loop code, but TS can't prove that.

* fix: add cleanupSession to error paths and remove unreachable code

P1 fixes (Greptile review):
- cleanupSession called on fetch error throw path
- cleanupSession called on non-retryable HTTP error throw path
- Removed unreachable post-loop code (loop always terminates via return or throw)

Adds explicit post-loop throw to satisfy TypeScript strict return check.

* fix: address Greptile review feedback on concurrent session controls

Removes client-side session tracking that leaked on normal completion paths.

The session tracking was redundant — the server-side health check already reads from

sessionRuntime.getSessionActivitySnapshot() for busy-session detection.

Changes:

- Remove activeSessions Set and all session-tracking functions from provider-tracker

- Remove trackSessionStarted/cleanupSession calls from client.ts

- Remove unreachable (response as Response) block after retry loop

- Make upstreamStallTimeoutMs conditional: 60s when >1 sessions, 20s otherwise

Refs #1069

* fix: enforce dynamic concurrency safeguards

---------

Co-authored-by: Bohdan Triapitsyn <artmore@protonmail.com>

packages/web/server/lib/event-stream/upstream-reader.test.js

@@ -117,6 +117,51 @@ describe('createUpstreamSseReader', () => {
     expect(reader.getLastEventId()).toBe('evt-2');
   });
 
+  it('resolves the stall timeout for each upstream read window', async () => {
+    const events = [];
+    let attempt = 0;
+    let currentTimeout = 10;
+    let reader;
+
+    reader = createUpstreamSseReader({
+      buildUrl: () => 'http://127.0.0.1:4096/global/event',
+      stallTimeoutMs: () => currentTimeout,
+      reconnectDelayMs: 0,
+      fetchImpl: async (_url, options) => {
+        attempt += 1;
+
+        if (attempt === 1) {
+          currentTimeout = 60;
+          return createSseResponse({
+            signal: options.signal,
+            holdOpen: true,
+            blocks: [
+              'id: evt-1\ndata: {"type":"server.connected","properties":{}}\n\n',
+            ],
+          });
+        }
+
+        return createSseResponse({
+          signal: options.signal,
+          blocks: [
+            'id: evt-2\ndata: {"type":"session.updated","properties":{}}\n\n',
+          ],
+        });
+      },
+      onEvent(event) {
+        events.push(event.eventId);
+        if (event.eventId === 'evt-2') {
+          reader.stop();
+        }
+      },
+    });
+
+    await reader.start();
+
+    expect(events).toEqual(['evt-1', 'evt-2']);
+    expect(attempt).toBe(2);
+  });
+
   it('reports unavailable upstream responses and continues reconnecting until stopped', async () => {
     const errors = [];
     let attempt = 0;