fix(json): preserve fenced structured replies

Author: jdalton

src/json.mts

@@ -84,6 +84,9 @@ export function isParseableJson(text: string): boolean {
 export function mergePrefill(prefill: string, raw: string): string {
   const trimmed = raw.trimStart()
   const trimmedPrefill = prefill.trimEnd()
+  if (extractJsonFence(trimmed) !== undefined) {
+    return raw
+  }
   if (trimmed.startsWith(trimmedPrefill)) {
     return raw
   }

test/json.test.mts

@@ -46,6 +46,11 @@ describe('json', () => {
     expect(mergePrefill('{"a":', '{"a":1}')).toBe('{"a":1}')
   })
 
+  it('does not prepend a prefill to a complete fenced response', () => {
+    const raw = '```json\n{"a":1}\n```'
+    expect(mergePrefill('{"a":', raw)).toBe(raw)
+  })
+
   it('wraps an array-element continuation of a nested-array prefill', () => {
     // The model continued from `{"updates":[` without echoing it, so raw alone
     // is unbalanced (`{…}]}`) but prefill+raw parses.