fix: make the sampling invariant test actually enforce the constraint

The test passed overlay and sampling as separate objects while production
aliases them, so an in-place mutation slipped through it. Also syncs the
telemetry schema page and adds the missing postgres round-trip assertion.

tests/integration/test_postgres_telemetry.py

@@ -11,6 +11,7 @@ DSN 走 .env `PGW_TELEMETRY_PG_DSN`,缺则 skip。该实例上有 app/chs_prod
 from __future__ import annotations
 
 import asyncio
+import json
 import os
 from uuid import uuid4
 
@@ -180,9 +181,12 @@ class TestObservabilityColumns:
             await _record_minimal(recorder, call_id=_cid("hit"), cached_prompt_tokens=64)
             await _record_minimal(recorder, call_id=_cid("zero"), cached_prompt_tokens=0)
             await _record_minimal(recorder, call_id=_cid("model"), model_reported="MiniMax-01")
+            await _record_minimal(
+                recorder, call_id=_cid("samp"), sampling='{"seed": 42, "temperature": 0}'
+            )
             rows = await _fetch(
                 dsn,
-                "SELECT call_id, cached_prompt_tokens, model_reported FROM llm_calls "
+                "SELECT call_id, cached_prompt_tokens, model_reported, sampling FROM llm_calls "
                 "WHERE call_id LIKE $1",
                 f"{_RUN_PREFIX}-%",
             )
@@ -191,6 +195,9 @@ class TestObservabilityColumns:
             assert by_id[_cid("zero")]["cached_prompt_tokens"] == 0  # 真实零命中 ≠ NULL
             assert by_id[_cid("model")]["cached_prompt_tokens"] is None
             assert by_id[_cid("model")]["model_reported"] == "MiniMax-01"
+            # issue #4: PG 侧也须验非空 sampling 能读回原值(不只是列存在)
+            assert json.loads(by_id[_cid("samp")]["sampling"]) == {"seed": 42, "temperature": 0}
+            assert by_id[_cid("hit")]["sampling"] is None
         finally:
             await recorder.aclose()