fix(ai): surface provider HTTP error body instead of opaque SDK message

Add a shared normalizeProviderError helper in packages/ai/src/utils/error-body.ts
and route the 8 body-blind / status-only providers through it (amazon-bedrock,
azure-openai-responses, google, google-vertex, images/openrouter,
openai-codex-responses, openai-completions, openai-responses). Non-schema 4xx/5xx
responses from proxies / gateways now show the real reason carried in the response
body alongside the HTTP status, instead of "403 status code (no body)" or
"Unknown: UnknownError".

The helper probes status (statusCode, status, $metadata.httpStatusCode,
$response.statusCode) and body (body, parsed error object, $response.body) across
the Mistral, OpenAI, Google, and Bedrock SDK shapes, truncates the body at a 4000
char cap, and preserves error.message when the SDK already folded the body in
(Anthropic / Google happy path). mistral.ts and anthropic.ts are left untouched.
Provider prefixes and the OpenRouter metadata.raw append are preserved.

closes #5763

packages/ai/src/api/openai-responses.ts

@@ -15,6 +15,7 @@ import type {
 	StreamOptions,
 	Usage,
 } from "../types.ts";
+import { formatProviderError, normalizeProviderError } from "../utils/error-body.ts";
 import { AssistantMessageEventStream } from "../utils/event-stream.ts";
 import { headersToRecord } from "../utils/headers.ts";
 import { getProviderEnvValue } from "../utils/provider-env.ts";
@@ -70,19 +71,7 @@ function getPromptCacheRetention(
 }
 
 function formatOpenAIResponsesError(error: unknown): string {
-	if (error instanceof Error) {
-		const status = (error as Error & { status?: unknown }).status;
-		const statusCode = typeof status === "number" ? status : undefined;
-		if (statusCode !== undefined) {
-			return `OpenAI API error (${statusCode}): ${error.message}`;
-		}
-		return error.message;
-	}
-	try {
-		return JSON.stringify(error);
-	} catch {
-		return String(error);
-	}
+	return formatProviderError(normalizeProviderError(error), "OpenAI API error");
 }
 
 // OpenAI Responses-specific options