chore(stream-check): update default health check models to latest

Replaces deprecated gpt-5.1-codex@low with gpt-5.4@low and switches
the Gemini default from gemini-3-pro-preview to gemini-3-flash-preview
to pick the lightest variant of the latest series for fast, low-cost
health checks.

https://claude.ai/code/session_01NGWLchcTP76rJHjiP5Ehte

src-tauri/src/services/stream_check.rs

@@ -55,8 +55,8 @@ impl Default for StreamCheckConfig {
             max_retries: 2,
             degraded_threshold_ms: 6000,
             claude_model: "claude-haiku-4-5-20251001".to_string(),
-            codex_model: "gpt-5.1-codex@low".to_string(),
-            gemini_model: "gemini-3-pro-preview".to_string(),
+            codex_model: "gpt-5.4@low".to_string(),
+            gemini_model: "gemini-3-flash-preview".to_string(),
             test_prompt: default_test_prompt(),
         }
     }

src/components/usage/ModelTestConfigPanel.tsx

@@ -25,7 +25,7 @@ export function ModelTestConfigPanel() {
     degradedThresholdMs: "6000",
     claudeModel: "claude-haiku-4-5-20251001",
     codexModel: "gpt-5.4@low",
-    geminiModel: "gemini-3-pro-preview",
+    geminiModel: "gemini-3-flash-preview",
     testPrompt: "Who are you?",
   });