feat: revamp DeepSeek v4 model handling

- replace legacy DeepSeek ids with the new deepseek-v4 model family\n- move thinking control to request parameters and preserve assistant reasoning content\n- switch history split to IGNORE transcript injection and map upload auth failures to 401\n- update admin defaults, API docs, samples, and tests for the new model scheme

tests/scripts/capture-raw-stream-sample.sh

@@ -7,7 +7,7 @@ cd "$ROOT_DIR"
 CONFIG_PATH="${1:-config.json}"
 SAMPLE_ID="${2:-capture-$(date -u +%Y%m%dT%H%M%SZ)}"
 QUESTION="${3:-广州天气}"
-MODEL="${4:-deepseek-reasoner-search}"
+MODEL="${4:-deepseek-v4-pro-search}"
 API_KEY="${5:-}"
 ADMIN_KEY="${DS2API_ADMIN_KEY:-admin}"