fix(tangle): truncate oversized tool output instead of discarding a paid turn

[drewstone]

Follow-up to #311. That PR removed the instance; this one removes the class.

#311 left a 4× mismatch

It widened tool output from the 16 KiB metadata bound to the 1 MiB content bound. But the producer still serializes each tool value up to 4 MiB, and the consumer bound is 1 MiB across the whole record, not per value. Measured against the shipped 1.1.9:

0.9 MiB x1                    ACCEPTED
2 MiB x1 (producer allows 4)  REJECTED — Tangle prompt result exceeded its JSON bound
0.6 MiB x2 = 1.2 MiB total    REJECTED — Tangle prompt result exceeded its JSON bound

256× became 4×, and the multi-value case is worse than the single-value case.

The mismatch was never the real defect

This validator runs inside the terminal result read — after the live stream has drained and after the usage receipt has been credited. A throw here cannot prevent the work or the charge. It can only destroy a finished, fully paid turn, which a supervisor then reports as a child that did nothing at all.

That failure mode cost one Discovery Lab 143 of 199 children across 16 pursuits, and its six-stage sourcing graph blocked in 24 of 24 invocations.

Any bound that throws on this path is one large page away from doing it again. Widening it is not a fix; it is a bigger margin before the same outcome.

Change

Oversized tool output is replaced, not refused. Each pass cuts the widest toolInvocations[].result by the record's measured overflow and leaves a marker naming both byte counts:

[truncated by the Tangle provider: kept 1046528 of 4194304 bytes to stay inside the 1048576-byte record content bound]

The caller still receives the turn, its response text, its usage, and every tool call it made. What it loses is the tail of an oversized result, marked where it was cut, so a reader can tell truncation from a tool that genuinely returned little and can choose to re-fetch.

Only tool output is truncatable. Every other field is identity, accounting, or control material where a silently shortened value would be worse than a refusal — so a record whose overflow is elsewhere still refuses, as does one breaking the node, depth, or array limits, which trimming a string cannot satisfy.

Tests

One added, fails on 1.1.9. It pins: a 2 MiB single result survives; the producer's own 4 MiB per-value maximum survives; several results that each fit but together do not converge rather than the first being destroyed; the marker carries both byte counts; every tool call is still present; and an oversized response still throws.

Provider suite 264 pass, typecheck clean. 1.1.9 → 1.1.10.

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