feat: wire the telemetry text cap through settings

`PGW_TELEMETRY_TEXT_CAP` now reaches the emitter on every assembly path.
Unset means no truncation, which stays the default: a truncated row is
no longer audit evidence and cannot be replayed, and downstreams rely on
that today. The flip side — contracts and bids sitting in `llm_calls`
indefinitely, multi-tenant — is spelled out in `.env.example` so readers
can weigh both.

All three `from_settings` paths are wired (chat, embedding, OCR): they
write the same table, so capping only chat would leave half of it
uncontrolled. `TelemetryEmitter.__init__` now rejects `text_cap <= 0`;
it is the single point where the three clients converge, so the direct
construction path — a public assembly route the settings guard never
sees — is covered too. `0` would otherwise reduce every body to a bare
elision marker.

tests/unit/test_config.py

@@ -386,6 +386,33 @@ class TestTelemetrySchemaMode:
             )
 
 
+class TestTelemetryTextCap:
+    """`PGW_TELEMETRY_TEXT_CAP`(issue #12): 二态键,未设即不截断。
+
+    与 `PGW_TELEMETRY_SCHEMA_MODE` 的三态不同,这里"未设"本身就是最终答案
+    (不截断),没有需要按后端派生的第二种缺省,故不走 `_load_choice` 那套。
+    """
+
+    def test_unset_key_means_no_truncation(self):
+        """缺省不截断是人类决策: 截断后的遥测不再是审计证据、无法复现重放。"""
+        assert GatewaySettings.from_env("LLM", env=_env()).telemetry_text_cap is None
+
+    def test_positive_value_is_parsed_as_int(self):
+        s = GatewaySettings.from_env("LLM", env=_env(PGW_TELEMETRY_TEXT_CAP="2000"))
+        assert s.telemetry_text_cap == 2000
+
+    @pytest.mark.parametrize("raw", ["0", "-1"])
+    def test_non_positive_rejected(self, raw):
+        """0 会把每条正文退化成一个省略标记,负数无意义;都不是"不截断"的写法。"""
+        with pytest.raises(ValueError, match="PGW_TELEMETRY_TEXT_CAP"):
+            GatewaySettings.from_env("LLM", env=_env(PGW_TELEMETRY_TEXT_CAP=raw))
+
+    def test_non_integer_rejected_naming_the_env_key(self):
+        """报错须点出 env 键名: 这条路的调用方看得懂的是键名,不是字段名。"""
+        with pytest.raises(ValueError, match="PGW_TELEMETRY_TEXT_CAP"):
+            GatewaySettings.from_env("LLM", env=_env(PGW_TELEMETRY_TEXT_CAP="2k"))
+
+
 class TestOcrSettings:
     """M3 OcrSettings(设计 §3.4): 复用 GatewaySettings,无 OCR 专用键。"""
 
@@ -622,6 +649,12 @@ class TestCrossFieldInvariants:
 
     # —— 标量域 ——
 
+    def test_non_positive_text_cap_rejected(self):
+        """env 路只覆盖 from_env;直接构造与 replace 同样能把 0 传进来(issue #12)。"""
+        base = self._base()
+        with pytest.raises(ValueError, match="telemetry_text_cap"):
+            dataclasses.replace(base, telemetry_text_cap=0)
+
     def test_negative_structured_retries_rejected(self):
         base = self._base()
         with pytest.raises(ValueError, match="structured_max_retries"):