fix: cap uncapped string accumulators (RangeError: Invalid string length)

[jpr5]

Problem

RangeError: Invalid string length was firing ~1/sec in prod (caught at server.ts:1230). V8's max string length on 64-bit is 2^29-1 (~512MiB of UTF-16 code units); appending past it throws.

Two uncapped in-memory string accumulators in the aimock binary build a JS string past that limit:

1. src/agui-recorder.ts — on stream end the recorder does Buffer.concat(chunks).toString() over every buffered upstream SSE chunk with no byte cap (unlike the generic proxy path in recorder.ts, which is capped at 64MiB maxProxyBufferBytes / 256MiB PROXY_BUFFER_HARD_CEILING).
2. src/stream-collapse.ts — the content += delta / reasoning += delta / entry.arguments += delta / audioB64 += … / argsStr += … accumulators (plus orderAtoms coalescing) grow unbounded across every collapser.

Amplified 07-13 by the real-key fixture-miss passthrough (b2ef945): large real upstream responses now flow into these accumulators. The recorder.ts comment claiming "stream-collapse never throws Invalid string length" was false for these paths and is fixed here.

Fix

Mirrors the existing PROXY_BUFFER_HARD_CEILING / maxProxyBufferBytes discipline — bound then mark truncated, never fail the client. The client always receives its full response; only the recording is degraded to a clean skip.

• agui-recorder.ts: new configurable record-buffer cap (AGUIRecordConfig.maxRecordBufferBytes, default 64MiB, clamped to a 256MiB AGUI_RECORD_BUFFER_HARD_CEILING). On trip: stop accumulating, free the buffer, skip fixture construction, keep the live client relay untouched — never Buffer.concat().toString() the oversized buffer.
• stream-collapse.ts: guard each collapser's input under MAX_COLLAPSE_STRING_LENGTH (256Mi code units) before accumulation. The sum of any single accumulator can never exceed the total input, so bounding the input bounds every accumulator at one chokepoint (no per-+= whack-a-mole). Over-ceiling input is truncated and the result stamped truncated: true so the recorder skips journaling a partial fixture. Also covers the exported direct-call collapse entry points where no byte cap ran.
• Fixed the false recorder.ts:31 comment.
• Test-only overrides (setAGUIRecordBufferCeilingForTests / setCollapseStringLimitForTests) let the cap suites exercise truncation without allocating hundreds of MB.

Local red-green proof (real failure surface)

RED — AG-UI recorder (streaming ~535MiB of real SSE through proxyAndRecordAGUI on the unmodified build):

[repro] streaming ~535 MiB through the AG-UI recorder…
[aimock] NO AG-UI FIXTURE MATCH — proxying to http://127.0.0.1:49292
[RED] uncaughtException in recorder: Error: Cannot create a string longer than 0x1fffffe8 characters
RED EXIT: 1

(0x1fffffe8 = 2^29-1 — the exact V8 max-string-length boundary, i.e. the prod Invalid string length.)

GREEN — same repro, post-fix:

[repro] streaming ~535 MiB through the AG-UI recorder…
[aimock] AG-UI upstream response exceeded the 67108864-byte record buffer cap — relaying full body to client, but skipping fixture recording to bound memory
[aimock] AG-UI record buffer cap exceeded — response relayed to client, recording skipped
[repro] client got status=200, bytesReceived=562041225
[GREEN] client received full body and recorder did not throw

Client received all 562,041,225 bytes — relay unaffected, only recording skipped.

stream-collapse mutation-guard (real collapseOpenAISSE, input just over the 256Mi ceiling):

• pre-fix: content.length=276824064, truncated=undefined (unbounded, no clamp)
• post-fix: content.length=260046848, truncated=true (clamped at ceiling, flagged)

Tests

• src/__tests__/agui-record-buffer-cap.test.ts (2) — full relay + no crash + recording skipped over cap; normal recording under cap.
• src/__tests__/stream-collapse-string-cap.test.ts (6) — every collapser clamps + flags truncated, never throws; no truncation under cap; Bedrock byte-cap.
• Full suite: 4501 passed (153 files). Lint + prettier + build + tsc clean.

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commit: 1d08625