feat(stream-check): refresh default models and detect model-not-found errors (#2099)

* 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

* feat(stream-check): detect model-not-found errors with dedicated toast

Health check previously classified failures purely by HTTP status code,
which meant deprecated/invalid models showed up as a generic "Not found
(404)" error pointing users to check the Base URL — misleading when the
URL is fine and only the test model is wrong (e.g. gpt-5.1-codex after
it was retired).

Backend: add detect_error_category() that inspects 4xx response bodies
for model-not-found indicators (model_not_found, does not exist,
invalid model, not_found_error, etc.) and returns a "modelNotFound"
category. Thread the resolved test model through build_stream_check_result
so the failed result carries it in model_used. Add StreamCheckResult
.error_category field (serde-skipped when None).

Frontend: useStreamCheck branches on errorCategory === "modelNotFound"
before the HTTP-status fallback and renders a toast.error with the model
name and a description pointing to Model Test Config. Add i18n keys
(modelNotFound / modelNotFoundHint) for zh/en/ja.

Tests: unit-test detect_error_category against real OpenAI/Anthropic
error shapes, 5xx false-positive avoidance, and plain 401 auth errors.

https://claude.ai/code/session_01NGWLchcTP76rJHjiP5Ehte

* fix(stream-check): add missing error_category field in fallback

The error_category field was added to StreamCheckResult in this branch
but the fallback constructor in stream_check_all_providers was not
updated, which broke cargo build.

---------

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

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?",
   });