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.

src/polygateway/client.py

@@ -136,6 +136,7 @@ class GatewayClient:
         quota_full: str = "wait",
         telemetry: TelemetryRecorder | None = None,
         pricing: PricingTable | None = None,
+        text_cap: int | None = None,
         cache: CacheBackend | None = None,
         cache_namespace: str | None = None,
         cache_ttl_s: int | None = None,
@@ -146,7 +147,11 @@ class GatewayClient:
         sleep: Any = asyncio.sleep,
         rng: Any = random.random,
     ) -> None:
-        emitter = TelemetryEmitter(telemetry, pricing=pricing) if telemetry is not None else None
+        emitter = (
+            TelemetryEmitter(telemetry, pricing=pricing, text_cap=text_cap)
+            if telemetry is not None
+            else None
+        )
         terminal = RetryMW(
             scope=scope,
             sources=sources,