feat(cache-fingerprint): 新增字段级指纹，支持逐字段 sha256 和字节数追踪

在请求缓存指纹中为每个 cache_relevant 字段单独计算 sha256 和字节数，
便于精确定位哪些字段发生了变化。fingerprint version 升级为 2。

src/services/provider/cache_fingerprint.py

@@ -101,6 +101,22 @@ def _extract_cache_relevant_payload(
     return subset, sorted(subset.keys())
 
 
+def _build_field_fingerprints(payload: Any, field_names: list[str]) -> dict[str, dict[str, Any]]:
+    if not isinstance(payload, Mapping) or not field_names:
+        return {}
+
+    field_fingerprints: dict[str, dict[str, Any]] = {}
+    for field_name in field_names:
+        if field_name not in payload:
+            continue
+        field_sha256, field_bytes = _hash_json_payload(payload[field_name])
+        field_fingerprints[field_name] = {
+            "sha256": field_sha256,
+            "bytes": field_bytes,
+        }
+    return field_fingerprints
+
+
 def build_request_cache_fingerprint(
     provider_request_body: Any,
     *,
@@ -117,13 +133,14 @@ def build_request_cache_fingerprint(
         normalized_format,
     )
     cache_relevant_sha256, cache_relevant_bytes = _hash_json_payload(cache_relevant_payload)
+    field_fingerprints = _build_field_fingerprints(cache_relevant_payload, cache_relevant_keys)
 
     top_level_keys = []
     if isinstance(provider_request_body, Mapping):
         top_level_keys = sorted(str(key) for key in provider_request_body.keys())
 
     fingerprint: dict[str, Any] = {
-        "version": 1,
+        "version": 2,
         "provider_api_format": normalized_format,
         "payload_sha256": payload_sha256,
         "payload_bytes": payload_bytes,
@@ -131,6 +148,7 @@ def build_request_cache_fingerprint(
         "cache_relevant_bytes": cache_relevant_bytes,
         "top_level_keys": top_level_keys,
         "cache_relevant_keys": cache_relevant_keys,
+        "field_fingerprints": field_fingerprints,
     }
 
     prompt_cache_key = _get_prompt_cache_key(provider_request_body)

tests/services/test_request_cache_fingerprint.py

@@ -65,8 +65,10 @@ def test_build_request_cache_fingerprint_is_stable_for_dict_key_reordering() ->
 
     assert fingerprint_a is not None
     assert fingerprint_b is not None
+    assert fingerprint_a["version"] == 2
     assert fingerprint_a["payload_sha256"] == fingerprint_b["payload_sha256"]
     assert fingerprint_a["cache_relevant_sha256"] == fingerprint_b["cache_relevant_sha256"]
+    assert fingerprint_a["field_fingerprints"] == fingerprint_b["field_fingerprints"]
     assert fingerprint_a["prompt_cache_key"] == "pcache-123"
     assert fingerprint_a["cache_relevant_keys"] == [
         "input",
@@ -75,6 +77,8 @@ def test_build_request_cache_fingerprint_is_stable_for_dict_key_reordering() ->
         "prompt_cache_key",
         "tools",
     ]
+    assert fingerprint_a["field_fingerprints"]["instructions"]["bytes"] > 0
+    assert fingerprint_a["field_fingerprints"]["tools"]["bytes"] > 0
 
 
 def test_build_request_cache_fingerprint_ignores_non_prompt_fields_in_cache_hash() -> None:
@@ -89,6 +93,8 @@ def test_build_request_cache_fingerprint_ignores_non_prompt_fields_in_cache_hash
     assert fingerprint_b is not None
     assert fingerprint_a["payload_sha256"] != fingerprint_b["payload_sha256"]
     assert fingerprint_a["cache_relevant_sha256"] == fingerprint_b["cache_relevant_sha256"]
+    assert fingerprint_a["field_fingerprints"] == fingerprint_b["field_fingerprints"]
+    assert "temperature" not in fingerprint_a["field_fingerprints"]
 
 
 def test_build_request_cache_fingerprint_tracks_prompt_changes() -> None:
@@ -102,6 +108,18 @@ def test_build_request_cache_fingerprint_tracks_prompt_changes() -> None:
     assert fingerprint_a is not None
     assert fingerprint_b is not None
     assert fingerprint_a["cache_relevant_sha256"] != fingerprint_b["cache_relevant_sha256"]
+    assert (
+        fingerprint_a["field_fingerprints"]["instructions"]["sha256"]
+        != fingerprint_b["field_fingerprints"]["instructions"]["sha256"]
+    )
+    assert (
+        fingerprint_a["field_fingerprints"]["input"]["sha256"]
+        == fingerprint_b["field_fingerprints"]["input"]["sha256"]
+    )
+    assert (
+        fingerprint_a["field_fingerprints"]["tools"]["sha256"]
+        == fingerprint_b["field_fingerprints"]["tools"]["sha256"]
+    )
 
 
 def test_sanitize_request_metadata_preserves_cache_fingerprint(
@@ -163,8 +181,10 @@ async def test_message_telemetry_record_success_keeps_response_shape_and_adds_fi
     metadata = captured["metadata"]
     assert metadata["model_version"] == "gpt-5.4-2026-03-01"
     assert "response" not in metadata
+    assert metadata["cache_fingerprint"]["version"] == 2
     assert metadata["cache_fingerprint"]["provider_api_format"] == "openai:cli"
     assert metadata["cache_fingerprint"]["prompt_cache_key"] == "pcache-123"
+    assert "instructions" in metadata["cache_fingerprint"]["field_fingerprints"]
 
 
 @pytest.mark.asyncio
@@ -203,5 +223,7 @@ async def test_message_telemetry_record_failure_keeps_request_metadata_and_adds_
 
     metadata = captured["metadata"]
     assert metadata["perf"]["ttfb_ms"] == 12
+    assert metadata["cache_fingerprint"]["version"] == 2
     assert metadata["cache_fingerprint"]["provider_api_format"] == "openai:cli"
     assert metadata["cache_fingerprint"]["prompt_cache_key"] == "pcache-123"
+    assert "input" in metadata["cache_fingerprint"]["field_fingerprints"]