refactor: drop the now-unused source parameter from usage resolvers

三态兜底不再读源配置,_resolve_usage / _resolve_stream_usage /
_resolve_embedding_usage 的 source 形参已成死参数;保留它等于在签名上继续
宣称用量口径依赖源配置,与本次改动切断该依赖的意图相悖。同步三个调用点
与测试的直接调用;SourceConfig 仍被文件内错误翻译等函数使用,import 保留。

tests/unit/test_usage_source_domain.py

@@ -80,7 +80,7 @@ class _MemoryRecorder:
     ],
 )
 def test_resolve_usage_stays_in_domain(usage):
-    assert _resolve_usage(usage, _src())[2] in USAGE_SOURCES
+    assert _resolve_usage(usage)[2] in USAGE_SOURCES
 
 
 @pytest.mark.parametrize(
@@ -95,7 +95,7 @@ def test_resolve_usage_stays_in_domain(usage):
     ],
 )
 def test_resolve_embedding_usage_stays_in_domain(data):
-    assert _resolve_embedding_usage(data, _src())[1] in USAGE_SOURCES
+    assert _resolve_embedding_usage(data)[1] in USAGE_SOURCES
 
 
 def _sse(*frames, done):