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_openai_compat.py

@@ -113,7 +113,7 @@ async def _recorded_cost(result, source):
         source_name=source.name,
         usage_source=result.usage_source,
     )
-    await TelemetryEmitter(recorder, pricing=_PRICING).emit_attempt(
+    await TelemetryEmitter(recorder, pricing=_PRICING, text_cap=None).emit_attempt(
         request=ChatRequest(messages=[{"role": "user", "content": "hi"}]),
         source=source,
         call_id="cid-1",