fix: always drop orphaned OpenAI reasoning blocks in session history

downgradeOpenAIReasoningBlocks was only called on model change, but
orphaned reasoning items (e.g. from an aborted stream) can exist without
a model switch and cause a 400 from the OpenAI Responses API.

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

src/agents/pi-embedded-runner.sanitize-session-history.test.ts

@@ -204,7 +204,7 @@ describe("sanitizeSessionHistory", () => {
     expect(result.map((msg) => msg.role)).toEqual(["user"]);
   });
 
-  it("does not downgrade openai reasoning when the model has not changed", async () => {
+  it("downgrades orphaned openai reasoning even when the model has not changed", async () => {
     const sessionEntries = [
       makeModelSnapshotEntry({
         provider: "openai",
@@ -222,7 +222,7 @@ describe("sanitizeSessionHistory", () => {
       sessionManager,
     });
 
-    expect(result).toEqual(messages);
+    expect(result).toEqual([]);
   });
 
   it("downgrades openai reasoning only when the model changes", async () => {

src/agents/pi-embedded-runner/google.ts

@@ -457,10 +457,9 @@ export async function sanitizeSessionHistory(params: {
         modelId: params.modelId,
       })
     : false;
-  const sanitizedOpenAI =
-    isOpenAIResponsesApi && modelChanged
-      ? downgradeOpenAIReasoningBlocks(sanitizedToolResults)
-      : sanitizedToolResults;
+  const sanitizedOpenAI = isOpenAIResponsesApi
+    ? downgradeOpenAIReasoningBlocks(sanitizedToolResults)
+    : sanitizedToolResults;
 
   if (hasSnapshot && (!priorSnapshot || modelChanged)) {
     appendModelSnapshot(params.sessionManager, {