feat: cap telemetry bodies at a configurable length

Chat rows stored full message and response text with no upper bound, so
downstream contracts and tenders lived in llm_calls indefinitely. Add
_cap_text/_cap_messages in the single telemetry exit (_record), applied
after digest_messages and before json.dumps, plus to response/thinking.

Capping is per text, not over the serialized JSON: cutting the whole
string would emit invalid JSON into an unvalidated TEXT column. The cap
builds new dicts and never mutates in place — digest_messages passes
non-list content straight through as the same object, so an in-place cut
would silently poison the caller's messages and the cache key.

text_cap is required on TelemetryEmitter (internal class, three known
construction sites) and defaults to None on the three public clients, so
the default behaviour stays byte-for-byte identical. Settings wiring
lands separately.

tests/unit/test_usage_source_domain.py

@@ -254,7 +254,7 @@ def _resp(usage_source):
 @pytest.mark.parametrize("emitted", _DOMAIN)
 async def test_emit_attempt_success_stays_in_domain(emitted):
     recorder = _MemoryRecorder()
-    await TelemetryEmitter(recorder).emit_attempt(
+    await TelemetryEmitter(recorder, text_cap=None).emit_attempt(
         request=_REQ,
         source=_src(),
         call_id="cid",
@@ -268,7 +268,7 @@ async def test_emit_attempt_success_stays_in_domain(emitted):
 async def test_emit_attempt_failed_attempt_stays_in_domain():
     """失败尝试无 response,`usage_source` 取 emitter 自己的字面量。"""
     recorder = _MemoryRecorder()
-    await TelemetryEmitter(recorder).emit_attempt(
+    await TelemetryEmitter(recorder, text_cap=None).emit_attempt(
         request=_REQ,
         source=_src(),
         call_id="cid",
@@ -282,14 +282,16 @@ async def test_emit_attempt_failed_attempt_stays_in_domain():
 @pytest.mark.parametrize("emitted", _DOMAIN)
 async def test_emit_cache_hit_stays_in_domain(emitted):
     recorder = _MemoryRecorder()
-    await TelemetryEmitter(recorder).emit_cache_hit(request=_REQ, response=_resp(emitted))
+    await TelemetryEmitter(recorder, text_cap=None).emit_cache_hit(
+        request=_REQ, response=_resp(emitted)
+    )
     assert recorder.rows[0]["usage_source"] in USAGE_SOURCES
 
 
 async def test_emit_terminal_failure_stays_in_domain():
     """终态失败无具体源,`usage_source` 同样取 emitter 字面量。"""
     recorder = _MemoryRecorder()
-    await TelemetryEmitter(recorder).emit_terminal_failure(
+    await TelemetryEmitter(recorder, text_cap=None).emit_terminal_failure(
         request=_REQ, call_id="cid", latency_ms=10, error="cancelled"
     )
     assert recorder.rows[0]["usage_source"] in USAGE_SOURCES