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>

test-features.mjs

@@ -4331,7 +4331,7 @@ test("stream: /health block is additive and exposes the divergence counter", ()
 });
 
 // ── OpenAI Structured Outputs (response_format) — lib/structured-output.mjs ──
-import { detectStructuredOutput, validateJsonSchema, extractJsonPayload, structuredSystemInstruction, StructuredOutputError, resolveMaxAttempts } from "./lib/structured-output.mjs";
+import { detectStructuredOutput, validateJsonSchema, validateJsonSchemaSafe, extractJsonPayload, structuredSystemInstruction, StructuredOutputError, resolveMaxAttempts } from "./lib/structured-output.mjs";
 
 test("detectStructuredOutput: json_schema shape", () => {
   const d = detectStructuredOutput({ response_format: { type: "json_schema", json_schema: { name: "x", strict: true, schema: { type: "object" } } } });
@@ -4358,6 +4358,28 @@ test("cacheHash: structured marker isolates JSON requests from the conversationa
   assert.equal(plain, cacheHash("m", msgs, { keyId: "k" })); // unchanged for normal requests
 });
 
+// ── validateJsonSchemaSafe (#181): deep value must NOT crash the handler ─────
+// A recursive schema + a model reply nested ~thousands deep overflows the value-
+// depth recursion → RangeError → the handler used to surface a generic 500. The
+// safe façade turns it into a validation miss (→ retry → refusal). Mutation-proof:
+// replace the wrapper body with a bare `validateJsonSchema(...)` call and the deep
+// test throws instead of returning errors.
+test("validateJsonSchemaSafe: pathologically deep value → errors, never throws", () => {
+  const schema = { $defs: { node: { type: "object", properties: { child: { $ref: "#/$defs/node" } } } }, $ref: "#/$defs/node" };
+  let deep = {};
+  let cur = deep;
+  for (let i = 0; i < 6000; i++) { cur.child = {}; cur = cur.child; } // way past any stack limit
+  let out;
+  assert.doesNotThrow(() => { out = validateJsonSchemaSafe(deep, schema, "$", true); }, "must not throw a RangeError out to the handler");
+  assert.ok(Array.isArray(out) && out.length > 0, "returns a non-empty validation error, so the retry loop yields a refusal not a 500");
+});
+
+test("validateJsonSchemaSafe: well-formed value passes through unchanged (byte-identical to the raw validator)", () => {
+  const schema = { type: "object", required: ["name", "age"], properties: { name: { type: "string" }, age: { type: "integer" } } };
+  assert.deepEqual(validateJsonSchemaSafe({ name: "a", age: 3 }, schema), validateJsonSchema({ name: "a", age: 3 }, schema));
+  assert.deepEqual(validateJsonSchemaSafe({ name: "a" }, schema), validateJsonSchema({ name: "a" }, schema)); // error case matches too
+});
+
 test("validateJsonSchema: valid object passes", () => {
   assert.deepEqual(validateJsonSchema({ name: "a", age: 3 }, { type: "object", required: ["name", "age"], properties: { name: { type: "string" }, age: { type: "integer" } } }), []);
 });