fix(openrouter): respect enableThoughtSignatures setting for message signatures (#5318)

The addOpenRouterSignatures function was previously converting and appending message.signature to reasoning_details unconditionally, ignoring the `enableThoughtSignatures` setting.

This change adds a check for `enableThoughtSignatures` before converting message.signature, while still ensuring the original signature property is deleted to prevent API schema validation errors (HTTP 400).

src/prompt-converters.js

@@ -1425,7 +1425,9 @@ export function addOpenRouterSignatures(messages, model) {
             details.push(detail);
         };
         if (typeof message.signature === 'string') {
-            addDetail(message.signature);
+            if (enableThoughtSignatures) {
+                addDetail(message.signature);
+            }
             delete message.signature;
         }
         if (Array.isArray(message.tool_calls)) {