feat: record sampling parameters in telemetry (port 20 to 21 fields)

Each of the three emitter entry points has a pinned meaning: only the
attempt path has an effective source, so only it merges extra_body.

src/polygateway/telemetry/sqlite.py

@@ -36,13 +36,18 @@ CREATE TABLE IF NOT EXISTS llm_calls (
     cost               REAL,
     created_at         TEXT    NOT NULL DEFAULT (datetime('now')),
     cached_prompt_tokens INTEGER,
-    model_reported       TEXT
+    model_reported       TEXT,
+    sampling             TEXT
 );
 """
 
 # 新列必须排在 created_at 之后: 旧表只能经 ALTER 追加到末尾,新建库若把它们
 # 插在前面,两条路径的物理列序会分叉(列序断言测试无合规修法)。
-_BACKFILL_COLUMNS = (("cached_prompt_tokens", "INTEGER"), ("model_reported", "TEXT"))
+_BACKFILL_COLUMNS = (
+    ("cached_prompt_tokens", "INTEGER"),
+    ("model_reported", "TEXT"),
+    ("sampling", "TEXT"),
+)
 
 _COLUMNS = (
     "call_id",
@@ -65,6 +70,7 @@ _COLUMNS = (
     "cost",
     "cached_prompt_tokens",
     "model_reported",
+    "sampling",
 )
 
 _INSERT = (
@@ -120,7 +126,7 @@ class SQLiteRecorder:
                     logger.warning("SQLite 遥测补列失败(写入将逐行降级): {}", exc)
 
     async def record_llm_call(self, **fields: object) -> None:
-        """写一行遥测;字段集合即 20 字段冻结签名(ports.TelemetryRecorder)。"""
+        """写一行遥测;字段集合即 21 字段冻结签名(ports.TelemetryRecorder)。"""
         if self._conn is None:
             return
         row = tuple(fields[col] for col in _COLUMNS)