refactor(api_format): 重构为结构化的 (ApiFamily, EndpointKind) 标识体系

将扁平的 APIFormat 枚举 (CLAUDE, OPENAI, GEMINI, *_CLI) 重构为二维结构：
- ApiFamily: 协议族 (openai, claude, gemini)
- EndpointKind: 端点变体 (chat, cli, video, image)

新增 EndpointSignature 数据类和 `family:kind` 签名键工具函数，
统一 JSON dict/metrics/logs 中的字符串标识格式。

tests/unit/test_api_format_signature.py

@@ -0,0 +1,41 @@
+import pytest
+
+from src.core.api_format.enums import ApiFamily, EndpointKind
+from src.core.api_format.signature import (
+    EndpointSignature,
+    make_signature_key,
+    normalize_endpoint_signature,
+    normalize_signature_key,
+    parse_signature_key,
+)
+
+
+def test_make_signature_key_accepts_enums_and_strings() -> None:
+    assert make_signature_key(ApiFamily.OPENAI, EndpointKind.CHAT) == "openai:chat"
+    assert make_signature_key("OpenAI", "CHAT") == "openai:chat"
+
+
+def test_parse_signature_key_roundtrip() -> None:
+    sig = parse_signature_key("OpenAI:CHAT")
+    assert sig.api_family == ApiFamily.OPENAI
+    assert sig.endpoint_kind == EndpointKind.CHAT
+    assert sig.key == "openai:chat"
+
+
+def test_parse_signature_key_invalid_raises() -> None:
+    with pytest.raises(ValueError):
+        parse_signature_key("OPENAI")  # missing ':'
+
+
+def test_normalize_signature_key() -> None:
+    assert normalize_signature_key("  OpenAI:CHAT  ") == "openai:chat"
+
+
+def test_normalize_endpoint_signature_tuple_input() -> None:
+    sig = normalize_endpoint_signature((ApiFamily.GEMINI, EndpointKind.VIDEO))
+    assert sig == EndpointSignature(api_family=ApiFamily.GEMINI, endpoint_kind=EndpointKind.VIDEO)
+
+
+def test_normalize_endpoint_signature_invalid_string_returns_default() -> None:
+    default = EndpointSignature(api_family=ApiFamily.CLAUDE, endpoint_kind=EndpointKind.CHAT)
+    assert normalize_endpoint_signature("not-a-signature", default=default) == default