fix(structured): narrow validateJsonSchemaSafe to RangeError-only + drop unused import (review fold-in)

Reviewer of #184: the catch-all would silently mask a future genuine bug (e.g. a
TypeError from a malformed schema) as a validation miss. Narrowed to
`if (e instanceof RangeError) return [...]; throw e;` so only the #181 deep-nesting
overflow becomes a refusal; any other throw surfaces at error level as before. +1
test proving a non-RangeError (required:42 → TypeError) re-throws. Dropped the now-
unused raw `validateJsonSchema` import from server.mjs. Merged current main (incl.
#183) so CI runs the true post-merge tree. Suite 433/0.

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

lib/structured-output.mjs

@@ -221,7 +221,12 @@ export function validateJsonSchemaSafe(value, schema, path = "$", strict = false
   try {
     return validateJsonSchema(value, schema, path, strict, root);
   } catch (e) {
-    return [`${path}: schema validation aborted (${e instanceof RangeError ? "value nesting too deep" : "internal error"})`];
+    // Catch ONLY the deep-nesting stack overflow (the #181 vector) and turn it into a validation
+    // miss → retry → refusal, never a 500. Any OTHER throw is a genuine bug: re-throw it so it
+    // surfaces at error level instead of being silently masked as "did not validate" (reviewer
+    // finding — a catch-all would hide a future TypeError behind a warn-level structured_retry).
+    if (e instanceof RangeError) return [`${path}: schema validation aborted (value nesting too deep)`];
+    throw e;
   }
 }