fix(tools): parse-harden Do Better Legal ask against leaky fine-tune output

The dbn-legal-agent-v3 fine-tune (Track 1 / family) emits a labelled-prose
template — duplicate `answer:` prefixes, markdown-escaped underscores (`\_`),
and a trailing raw JSON blob — rather than the strict JSON the Azure/gpt-4o
path produces via response_format. decodeJsonObject() returned null on that
invalid JSON, so ask() dumped the entire raw blob into `answer`.

Fix at the parse layer (no upstream response_format change, to avoid fighting
the fine-tune's training):
- dbnToolsRepairJsonText(): strip fences, drop only invalid `\_`/`\*` escapes,
  then balanced-brace scan collecting every top-level {...} (longest first) to
  recover an appended JSON object. Shared by both gateways' decodeJsonObject(),
  so all JSON tools benefit.
- dbnToolsParseLabeledFields(): parse labelled-prose into real fields when no
  JSON decodes, tolerating escaped key names and collapsing duplicate prefixes.
- ask() null-fallback now builds clean structured fields from the parsed prose
  instead of dumping raw; what_remains_uncertain becomes a proper list.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

includes/AzureOpenAiGateway.php

@@ -148,26 +148,7 @@ final class DbnAzureOpenAiGateway
 
     public function decodeJsonObject(string $content): ?array
     {
-        $content = trim($content);
-        $content = (string)preg_replace('/^```(?:json)?\s*\n?/i', '', $content);
-        $content = (string)preg_replace('/\n?```\s*$/', '', $content);
-        $content = trim($content);
-
-        $decoded = json_decode($content, true);
-        if (is_array($decoded)) {
-            return $decoded;
-        }
-
-        $start = strpos($content, '{');
-        $end   = strrpos($content, '}');
-        if ($start !== false && $end !== false && $end > $start) {
-            $candidate = substr($content, $start, $end - $start + 1);
-            $decoded = json_decode($candidate, true);
-            if (is_array($decoded)) {
-                return $decoded;
-            }
-        }
-        return null;
+        return dbnToolsRepairJsonText($content);
     }
 
     private function postJson(string $url, array $payload, int $timeout): array