fix(ui): show token stats and simplify context window calculation

- Track messageInfoVersion in cache signature to rebuild when tokens arrive via SSE
- Read tokens from step-finish part directly (embedded in SSE events)
- Simplify available tokens to show full context window when no explicit input limit

packages/ui/src/stores/message-v2/session-info.ts

@@ -116,18 +116,11 @@ export function updateSessionInfo(instanceId: string, sessionId: string): void {
     // Prefer explicit input limits when provided by the API.
     // This is used by the UI "Avail" chip.
     contextAvailableTokens = modelInputLimit
-  }
-
-  if (!contextAvailableFromPrevious && contextAvailableTokens === null) {
-    if (contextWindow > 0) {
-      if (latestHasContextUsage && actualUsageTokens > 0) {
-        contextAvailableTokens = Math.max(contextWindow - (actualUsageTokens + outputBudget), 0)
-      } else {
-        contextAvailableTokens = contextWindow
-      }
-    } else {
-      contextAvailableTokens = null
-    }
+  } else if (contextWindow > 0) {
+    // When no explicit input limit, show full context window capacity.
+    contextAvailableTokens = contextWindow
+  } else {
+    contextAvailableTokens = null
   }
 
   setSessionInfoByInstance((prev) => {