fix(confidence-engine): sanitize reasoning error responses

app/api/cases/overview/route.js

@@ -55,7 +55,9 @@ async function post(request) {
         {
           success: false,
           stage: error.statusCode === 400 ? "request_validation" : "provider",
-          error: error.message ?? "Overview synthesis failed",
+          error: error.statusCode === 400
+            ? "Invalid overview request"
+            : "Reasoning request could not be completed.",
         },
         { status: error.statusCode }
       );

app/api/cases/start/route.js

@@ -48,14 +48,8 @@ async function post(request) {
     return Response.json(
       {
         success: false,
-        error: diagnostics.error,
-        validationErrors: result.validationErrors,
-        diagnostics: result.diagnostics,
-        analysisErrors: result.analysisErrors,
-        validationIssues: result.validationIssues,
-        providerApiPath: result.providerApiPath,
-        providerExecution: result.providerExecution,
-        rawResponse: result.rawResponse ?? undefined,
+        error: status === 400 ? diagnostics.error : "Reasoning request could not be completed.",
+        ...(status === 400 ? { validationErrors: result.validationErrors } : {}),
       },
       { status },
     );

app/api/cases/synthesis/route.js

@@ -57,7 +57,9 @@ async function post(request) {
         {
           success: false,
           stage: error.statusCode === 400 ? "request_validation" : "provider",
-          error: error.message ?? "Synthesis failed",
+          error: error.statusCode === 400
+            ? "Invalid synthesis request"
+            : "Reasoning request could not be completed.",
         },
         { status: error.statusCode }
       );