fix(structured): crash-safe validation façade — deep model reply → refusal, not a 500 (closes #181)

#153's cyclic-$ref guard caps the REF-chain depth but not the DATA depth:
validateJsonSchema recurses on the value's nesting (properties/items/additionalProps),
so a model reply nested ~2000+ levels overflowed the stack with a RangeError, which
handleChatCompletions caught as a generic HTTP 500 instead of the spec-correct refusal.
(Found in the #153 final review, filed as #181; ≤1 spawn, no crash, no client-only
trigger — the value always comes from the model reply.)

New exported validateJsonSchemaSafe() wraps the validator: ANY throw (the deep-data
RangeError, or any future recursion hazard) becomes a single validation error, so the
structured-output retry loop treats a pathological reply as "did not validate" →
refusal. A well-formed reply is byte-identical (passes the inner errors through).
runStructuredCompletion calls the safe façade.

Chose the wrapper over threading a data-depth counter through six recursive call
sites: it protects every internal path at once (impossible to miss one) and stays
deterministically testable — a 6000-deep fixture reliably overflows on any platform.

Tests: +2, mutation-proven (revert the wrapper to a bare call → the deep test throws
RED). Suite 431/0. Rule 2: OCP-internal validation, no wire change, no cli.js citation.

Closes #181

Co-Authored-By: Claude <claude-opus-4-8> <noreply@anthropic.com>

lib/structured-output.mjs

@@ -209,6 +209,22 @@ export function validateJsonSchema(value, schema, path = "$", strict = false, ro
   return errors;
 }
 
+// Crash-safe façade over validateJsonSchema (issue #181). The validator recurses on the DATA's
+// nesting depth (properties/items/additionalProperties), which the REF_DEPTH_CAP does NOT bound —
+// only the ref-chain is. A model reply nested ~2000 levels deep therefore overflowed the stack with
+// a RangeError, which the request handler caught as a generic HTTP 500 instead of the spec-correct
+// `refusal`. This wrapper converts ANY throw (the deep-data RangeError, or any future recursion
+// hazard) into a single validation error, so the structured-output retry loop treats a pathological
+// reply as "did not validate" → refusal — never a 500, never a crash. A well-formed reply is
+// unaffected: the inner validator returns and this just passes its errors through.
+export function validateJsonSchemaSafe(value, schema, path = "$", strict = false, root = schema) {
+  try {
+    return validateJsonSchema(value, schema, path, strict, root);
+  } catch (e) {
+    return [`${path}: schema validation aborted (${e instanceof RangeError ? "value nesting too deep" : "internal error"})`];
+  }
+}
+
 function tryJsonParse(s) {
   try { return { ok: true, value: JSON.parse(s) }; } catch { return { ok: false }; }
 }