chore(lint): combine nested if dans bookslm_routes (SIM102)

Fix final pour atteindre 'All checks passed!' sur ruff check backend/.
Le commit précédent avait raté ce fichier (modifié par --unsafe-fixes
mais non inclus dans le commit).

backend/bookslm_routes.py

@@ -109,13 +109,12 @@ async def api_bookslm_chat(
         try:
             cfg_name = DEFAULT_PROVIDER
             # Check if default provider is available
-            if cfg_name != "gemini" and cfg_name in PROVIDERS:
-                if not PROVIDERS[cfg_name].get("api_key"):
-                    # Find first available provider
-                    for pname, pcfg in PROVIDERS.items():
-                        if pcfg.get("api_key") and pname != "gemini":
-                            cfg_name = pname
-                            break
+            if cfg_name != "gemini" and cfg_name in PROVIDERS and not PROVIDERS[cfg_name].get("api_key"):
+                # Find first available provider
+                for pname, pcfg in PROVIDERS.items():
+                    if pcfg.get("api_key") and pname != "gemini":
+                        cfg_name = pname
+                        break
 
             if cfg_name == "gemini" and PROVIDERS.get("gemini", {}).get("api_key"):
                 response = await _call_gemini(user_prompt, system_prompt, temperature=0.3, max_tokens=4096)