fix(reasoning): preserve mixed plaintext and drop incompatible state (#10949, #10959) (#10961)

Validated on the combined batch board + this branch: 231/231 across chatcore-translation-paths, reasoning-cache, strip-reasoning-blobs, and both Responses translator suites. Pre-merge: propagated the #11110/#11129 summary:[] defaults into five assertions here (each commented with its PR) — without it this branch red against the tip, and as a bonus the merge drains the 4 reasoning reds that were live on the tip from those merges. Plaintext now wins over a coexisting opaque companion; opaque-only drops cleanly for plaintext targets; combos keep explicit Skip. Fixes #10949 and #10959. Thank you @jackjinke!

src/app/(dashboard)/dashboard/combos/page.tsx

@@ -3896,15 +3896,6 @@ function ComboFormModal({ isOpen, combo, onClose, onSave, activeProviders, combo
                           )}
                         </option>
                       </select>
-                      {config.reasoningTransportFallback !== "skip" && (
-                        <p className="text-[10px] text-amber-600 dark:text-amber-300 mt-1">
-                          {getI18nOrFallback(
-                            t,
-                            "reasoningTransportFallbackDropWarning",
-                            "May lose continuation context or cause tool-call continuations to fail."
-                          )}
-                        </p>
-                      )}
                     </div>
                     <div>
                       <FieldLabelWithHelp

src/sse/handlers/chat.ts

@@ -1851,7 +1851,7 @@ async function handleSingleModelChat(
             modelPinned: runtimeOptions?.modelPinned ?? false,
             routingComboId: runtimeOptions?.routingComboId ?? null,
             sessionAffinityKey: runtimeOptions.sessionAffinityKey ?? null,
-            reasoningTransportFallback: runtimeOptions.reasoningTransportFallback ?? "skip",
+            reasoningTransportFallback: runtimeOptions.reasoningTransportFallback ?? "drop",
             managedLease: runtimeOptions.managedLease ?? null,
           },
           runtimeOptions

src/sse/handlers/chatHelpers.ts

@@ -422,7 +422,7 @@ export async function executeChatWithBreaker({
   conversationId = null,
   modelPinned = false,
   routingComboId = null,
-  reasoningTransportFallback = "skip",
+  reasoningTransportFallback = "drop",
   sessionAffinityKey = null,
   managedLease = null,
 }: ExecuteChatWithBreakerOptions): Promise<ExecuteChatWithBreakerResult> {