fix(web): address session fork review findings

packages/host/apiproxy/tests/client-handler.spec.ts

@@ -111,6 +111,21 @@ describe('unary round trip', () => {
     expect(response.result).toEqual({ ok: true, value: { items: [{ sessionId: 's1', updatedAt: 7, running: false, blank: false }] } })
   })
 
+  it('routes session fork with its optional cut anchor through the wire', async () => {
+    let seen: RpcRequest<{ sessionId: SessionId; atSeq?: number }> | undefined
+    const api = scriptedApi({
+      sessions: {
+        fork: (request) => {
+          seen = request
+          return ok(request, { sessionId: sid('s-child') })
+        },
+      },
+    })
+    const response = await client(api).sessions.fork({ sessionId: sid('s-parent'), atSeq: 7 })
+    expect(seen?.payload).toEqual({ sessionId: 's-parent', atSeq: 7 })
+    expect(response.result).toEqual({ ok: true, value: { sessionId: 's-child' } })
+  })
+
   it('routes workspace rename, delete, and insertSessionBefore through the wire', async () => {
     const api = scriptedApi()
     const c = client(api)