修复流式 AI 请求 max_tokens 不足导致 JSON 截断

此前流式 650 tokens，只够 4 个文字字段。新增 predicted_max
等 6 个字段后，中文解读经常超出上限致 JSON 不完整，触发重试。
现提升至 900 tokens（主请求 1200），并更新测试断言。

tests/test_web_observability.py

@@ -385,7 +385,7 @@ def test_city_ai_stream_request_only_asks_provider_for_observation_read():
 
     user_payload = request_payload["messages"][1]["content"]
     assert request_payload["stream"] is True
-    assert request_payload["max_tokens"] <= 650
+    assert request_payload["max_tokens"] <= 900
     assert request_payload["max_tokens"] < scan_terminal_service.SCAN_AI_MAX_TOKENS
     assert '"required_json_keys": ["metar_read_zh", "metar_read_en", "predicted_max", "range_low", "range_high", "unit", "confidence", "final_judgment_zh", "final_judgment_en", "reasoning_zh", "reasoning_en"]' in user_payload
     assert "predicted_max" in user_payload

web/scan_terminal_service.py

@@ -188,14 +188,14 @@ SCAN_AI_MAX_TOKENS = _env_int(
 )
 SCAN_CITY_AI_MAX_TOKENS = _env_int(
     "POLYWEATHER_SCAN_CITY_AI_MAX_TOKENS",
-    650,
-    min_value=300,
+    1200,
+    min_value=400,
     max_value=64000,
 )
 SCAN_CITY_AI_STREAM_MAX_TOKENS = _env_int(
     "POLYWEATHER_SCAN_CITY_AI_STREAM_MAX_TOKENS",
-    min(SCAN_CITY_AI_MAX_TOKENS, 650),
-    min_value=300,
+    min(SCAN_CITY_AI_MAX_TOKENS, 900),
+    min_value=400,
     max_value=64000,
 )