fix: use max_completion_tokens instead of max_tokens for GPT-4o

Azure OpenAI GPT-4o and newer models require 'max_completion_tokens'
instead of 'max_tokens'. Fixed in all 3 services:
- chat_service.py (2 call sites)
- azure_ai.py (validation service)
- tenants/router.py (test connection endpoint)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/domains/tenants/router.py

@@ -193,7 +193,7 @@ async def test_tenant_ai_config(
             client.chat.completions.create(
                 model=deployment,
                 messages=[{"role": "user", "content": "ping"}],
-                max_tokens=5,
+                max_completion_tokens=5,
             )
             return {"ok": True}
         except Exception as exc: