feat: record stream end mode in telemetry

llm_calls gains a nullable stream_end TEXT column (auto backfill,
manual-mode trimming as in 1.3.8). TelemetryRecorder.record_llm_call
takes a new required keyword stream_end; the emitter reads it from the
response on attempt rows and writes NULL for failed, cache-hit and
terminal rows.

Red: 18 tests in tests/unit/test_telemetry.py and test_ports.py failed
against the pre-change source (missing column, 38-field signature,
stream_end absent from emitted rows).

tests/unit/test_ports.py

@@ -150,6 +150,7 @@ class _DummyRecorder:
         total_latency_ms,
         stall_ms,
         stall_count,
+        stream_end,
     ) -> None: ...
 
 
@@ -322,7 +323,7 @@ class TestTelemetryRecorderSignature:
             "attempts",
             "total_latency_ms",
         } <= set(params)
-        assert len(params) - 1 == 38  # 减掉 self
+        assert len(params) - 1 == 39  # 减掉 self
 
     @pytest.mark.parametrize(
         "name",
@@ -341,6 +342,7 @@ class TestTelemetryRecorderSignature:
             "error_body",
             "attempts",
             "total_latency_ms",
+            "stream_end",
         ],
     )
     def test_caller_dimensions_have_no_default(self, name):