fix(captain): handle GPT-4.1-mini structured output inconsistency

GPT-4.1-mini sometimes returns JSON as a string instead of structured
output when tools are present in the request. This adds a fallback
parser that extracts the first valid JSON object from the string.

- No impact on working cases (Hash returns immediately)
- Handles concatenated JSON responses like `{...}{...}`
- Safe fallback for future "Choose Your Own LLM" feature

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

enterprise/app/services/captain/assistant/agent_runner_service.rb

@@ -73,40 +73,39 @@ class Captain::Assistant::AgentRunnerService
 
   # Response formatting methods
   def process_agent_result(result)
-    # DEBUG LOGGING - remove after investigation
-    Rails.logger.info "[Captain V2 DEBUG] result.output class: #{result.output.class.name}"
-    Rails.logger.info "[Captain V2 DEBUG] result.output value: #{result.output.inspect}"
-
-    if result.output.is_a?(String)
-      Rails.logger.warn '[Captain V2 DEBUG] Output is STRING not Hash - schema may not be applied!'
-      Rails.logger.warn "[Captain V2 DEBUG] String content: #{result.output}"
-    end
-    # END DEBUG LOGGING
-
     response = format_response(result.output)
-
-    # Extract agent name from context
     response['agent_name'] = result.context&.dig(:current_agent)
-
     response
   end
 
   def format_response(output)
-    if output.is_a?(Hash)
-      Rails.logger.info "[Captain V2 DEBUG] format_response: Hash with keys #{output.keys}"
-      return output.with_indifferent_access
+    return output.with_indifferent_access if output.is_a?(Hash)
+
+    # GPT-4.1-mini sometimes returns JSON as a string instead of structured output
+    # when tools are present in the request. Parse the first valid JSON object.
+    if output.is_a?(String) && output.strip.start_with?('{')
+      parsed = parse_first_json_object(output)
+      return parsed.with_indifferent_access if parsed
     end
 
-    # DEBUG LOGGING - remove after investigation
-    Rails.logger.warn "[Captain V2 DEBUG] format_response: Non-hash output, type=#{output.class.name}"
-    Rails.logger.warn "[Captain V2 DEBUG] format_response: content=#{output}"
-    # END DEBUG LOGGING
-
     # Fallback for backwards compatibility
-    {
-      'response' => output.to_s,
-      'reasoning' => 'Processed by agent'
-    }
+    { 'response' => output.to_s, 'reasoning' => 'Processed by agent' }
+  end
+
+  def parse_first_json_object(str)
+    depth = 0
+    start_idx = str.index('{')
+    return nil unless start_idx
+
+    (start_idx...str.length).each do |i|
+      depth += 1 if str[i] == '{'
+      depth -= 1 if str[i] == '}'
+      return JSON.parse(str[start_idx..i]) if depth.zero?
+    end
+
+    nil
+  rescue JSON::ParserError
+    nil
   end
 
   def error_response(error_message)