fix: cap journal body at 64KB to prevent Invalid string length crash on GET /__aimock/journal

[jpr5]

Problem

GET /__aimock/journal serializes the full journal (up to 1000 entries) via JSON.stringify. When LLM request bodies are large (~530 KB each in prod-like agentic traffic), the aggregate JSON string exceeds V8's 512 MB limit → RangeError: Invalid string length. The catch-all logged only err.message, so no stack ever appeared in prod logs.

Real stack (locally reproduced):

[aimock] [DIAG] Unhandled RangeError on GET /__aimock/journal:
RangeError: Invalid string length
    at JSON.stringify (<anonymous>)
    at handleControlAPI (file:///Users/jpr5/dev/aimock/dist/server.js:177:16)
    at handleHttpRequest (file:///Users/jpr5/dev/aimock/dist/server.js:822:10)
    at Server.<anonymous> (file:///Users/jpr5/dev/aimock/dist/server.js:778:3)
    at Server.emit (node:events:508:20)
    at parserOnIncoming (node:_http_server:1216:12)
    at HTTPParser.parserOnHeadersComplete (node:_http_common:125:17)
[aimock] Unhandled request error: Invalid string length

Fix

Cap the retained request-body size at 64 KB per journal entry at the journal source (journal.add()). Bodies exceeding the cap are replaced with a truncation marker { "__aimock_truncated": true, "originalByteSize": N, "note": "..." }. This bounds aggregate journal memory to ~64 MB at maxEntries=1000 (well under 256 MB), and protects all downstream consumers of journal.getAll(), not just /journal.

Also unifies the catch-all error log to include err.stack unconditionally for production observability (removes [DIAG] debug tag from the diagnosis branch).

RED (pre-fix, crash confirmed locally)

$ curl -s http://127.0.0.1:4010/__aimock/journal > /tmp/response.txt
$ wc -c /tmp/response.txt
       0 /tmp/response.txt   # empty — crash returned 500

$ grep -A 10 "RangeError" /tmp/aimock-fix-red.log
[aimock] [DIAG] Unhandled RangeError on GET /__aimock/journal:
RangeError: Invalid string length
    at JSON.stringify (<anonymous>)
    at handleControlAPI (file:///Users/jpr5/dev/aimock/dist/server.js:177:16)
    at handleHttpRequest (file:///Users/jpr5/dev/aimock/dist/server.js:822:10)
[aimock] Unhandled request error: Invalid string length

Repro: 1000 × 530 KB POST bodies → GET /__aimock/journal → RangeError: Invalid string length

GREEN (post-fix, no crash)

$ curl -s http://127.0.0.1:4010/__aimock/journal | wc -c
  520001   # 520 KB of valid JSON — NOT empty

$ curl -s http://127.0.0.1:4010/__aimock/journal | python3 -c "import sys,json; d=json.load(sys.stdin); print(f'entries={len(d)}, ok')"
entries=1000, ok

$ grep "RangeError\|Invalid string length" /tmp/aimock-fix-green.log | wc -l
       0   # no errors

# Truncation markers present in entries:
# entry[0]: __aimock_truncated=True, originalByteSize=542854
# entry[1]: __aimock_truncated=True, originalByteSize=542854
# entry[2]: __aimock_truncated=True, originalByteSize=542854
# Total truncated: 1000/1000

Regression test

Added to src/__tests__/journal.test.ts — "journal body cap (ISL regression)" describe block with 3 tests:

1. replaces oversized bodies with a truncation marker — 100 KB body → marker stored
2. retains bodies that are within the 64 KB cap — small body → stored as-is
3. JSON.stringify(journal.getAll()) does not throw with 1000 large-body entries — 1000 × 100 KB bodies, must not throw

Pre-push checks

• format: pass
• lint: pass
• tests: 4548 passed (154 test files)
• build: pass

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⚠️ Draft — do not merge. aimock is deployed to prod on Railway. A human must authorize merge + deploy.

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commit: 38a5773