fix(ai): skip errored/aborted assistant messages in transform-messages

Fixes OpenAI Responses 400 error 'reasoning without following item' by
skipping errored/aborted assistant messages entirely rather than filtering
at the provider level. This covers openai-responses, openai-codex-responses,
and future providers.

Removes strictResponsesPairing compat option (no longer needed).

Closes #838

packages/ai/test/openai-responses-reasoning-replay-e2e.test.ts

@@ -0,0 +1,81 @@
+import { Type } from "@sinclair/typebox";
+import { describe, expect, it } from "vitest";
+import { getModel } from "../src/models.js";
+import { complete, getEnvApiKey } from "../src/stream.js";
+import type { AssistantMessage, Context, Message, Tool } from "../src/types.js";
+
+const testToolSchema = Type.Object({
+	value: Type.Number({ description: "A number to double" }),
+});
+
+const testTool: Tool<typeof testToolSchema> = {
+	name: "double_number",
+	description: "Doubles a number and returns the result",
+	parameters: testToolSchema,
+};
+
+describe.skipIf(!process.env.OPENAI_API_KEY)("OpenAI Responses reasoning replay e2e", () => {
+	it("skips reasoning-only history after an aborted turn", { retry: 2 }, async () => {
+		const model = getModel("openai", "gpt-5-mini");
+
+		const apiKey = getEnvApiKey("openai");
+		if (!apiKey) {
+			throw new Error("Missing OPENAI_API_KEY");
+		}
+
+		const userMessage: Message = {
+			role: "user",
+			content: "Use the double_number tool to double 21.",
+			timestamp: Date.now(),
+		};
+
+		const assistantResponse = await complete(
+			model,
+			{
+				systemPrompt: "You are a helpful assistant. Use the tool.",
+				messages: [userMessage],
+				tools: [testTool],
+			},
+			{
+				apiKey,
+				reasoningEffort: "high",
+			},
+		);
+
+		const thinkingBlock = assistantResponse.content.find(
+			(block) => block.type === "thinking" && block.thinkingSignature,
+		);
+		if (!thinkingBlock || thinkingBlock.type !== "thinking") {
+			throw new Error("Missing thinking signature from OpenAI Responses");
+		}
+
+		const corruptedAssistant: AssistantMessage = {
+			...assistantResponse,
+			content: [thinkingBlock],
+			stopReason: "aborted",
+		};
+
+		const followUp: Message = {
+			role: "user",
+			content: "Say hello to confirm you can continue.",
+			timestamp: Date.now(),
+		};
+
+		const context: Context = {
+			systemPrompt: "You are a helpful assistant.",
+			messages: [userMessage, corruptedAssistant, followUp],
+			tools: [testTool],
+		};
+
+		const response = await complete(model, context, {
+			apiKey,
+			reasoningEffort: "high",
+		});
+
+		// The key assertion: no 400 error from orphaned reasoning item
+		expect(response.stopReason, `Error: ${response.errorMessage}`).not.toBe("error");
+		expect(response.errorMessage).toBeFalsy();
+		// Model should respond (text or tool call)
+		expect(response.content.length).toBeGreaterThan(0);
+	});
+});