Fix Bedrock advocate synthesis: engine guard, response_format, Claude engine option

- DeepResearchAgent: engine guard now accepts dbn_legal_v3, claude_sonnet, claude_haiku
  (previously stripped these to azure_mini, breaking dbn_legal_v3 selection and
  preventing Claude engines from reaching the correct synthesis branch)
- DbnBedrockGateway: remove response_format=json_object from chat payload — LiteLLM
  converts this to a tool-use constraint for Bedrock, routing output into tool_calls
  instead of content (root cause of the {} empty brief)
- advocate.php: add Claude Sonnet 4.6 (AWS Bedrock) engine option
- account, billing, dashboard, nav, min-sak: pending UI/flow changes from prior sessions

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

includes/DeepResearchAgent.php

@@ -40,7 +40,7 @@ final class DbnDeepResearchAgent
     ): array {
         $seedQuery   = trim($seedQuery);
         $pastedText  = trim($pastedText);
-        $engine      = in_array($engine, ['azure_mini', 'azure_full', 'gpu'], true) ? $engine : 'azure_mini';
+        $engine      = in_array($engine, ['azure_mini', 'azure_full', 'gpu', 'dbn_legal', 'dbn_legal_v3', 'claude_sonnet', 'claude_haiku'], true) ? $engine : 'azure_mini';
         $language    = dbnToolsNormalizeUiLanguage($language);
 
         $controls = $this->normalizeControls($controls);
@@ -1221,7 +1221,7 @@ PROMPT;
     ): array {
         $seedQuery  = trim($seedQuery);
         $pastedText = trim($pastedText);
-        $engine     = in_array($engine, ['azure_mini', 'azure_full', 'gpu'], true) ? $engine : 'azure_mini';
+        $engine     = in_array($engine, ['azure_mini', 'azure_full', 'gpu', 'dbn_legal', 'dbn_legal_v3', 'claude_sonnet', 'claude_haiku'], true) ? $engine : 'azure_mini';
         $language   = dbnToolsNormalizeUiLanguage($language);
         $controls   = $this->normalizeControls($controls);