Add POST LRO test case with bytes response type

[Copilot]

Adds a new test case for POST LRO operations where the final response type is bytes, enabling SDK generators to validate binary response handling in long-running operations.

Changes

• lro.tsp: Added exportBinary operation using ArmResourceActionAsync with ExportBinaryResult returning @body data: bytes with application/octet-stream content type
• mockapi.ts: Added mock handlers for the LRO flow (initial 202, AAO polling, Location polling returning binary)
• helper.ts + assets/image.png: New helper module and test asset for binary response payload

Example TypeSpec Definition

model ExportBinaryResult {
  @header contentType: "application/octet-stream";
  @body data: bytes;
}

exportBinary is ArmResourceActionAsync<
  Order,
  ExportBinaryRequest,
  ExportBinaryResult,
  LroHeaders = ArmCombinedLroHeaders<FinalResult = ExportBinaryResult> &
    Azure.Core.Foundations.RetryAfterHeader
>;

The operation follows the standard ARM LRO pattern with both Location and Azure-AsyncOperation headers, returning binary content on final Location poll.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>[spector] Add a POST long running API whose response type is bytes</issue_title>
<issue_description>GOAL:

add test case in https://github.com/Azure/typespec-azure/blob/a68aa69f03652b819048638e0eb593478c35f2c6/packages/azure-http-specs/specs/azure/resource-manager/operation-templates/lro.tsp for POST LRO whose response type is bytes.

Context:
You could refer to playground :

import "@typespec/http";
import "@azure-tools/typespec-azure-core";
import "@azure-tools/typespec-azure-resource-manager";

using Http;
using Rest;
using Versioning;
using Azure.Core;
using Azure.ResourceManager;

@armProviderNamespace
@service(#{ title: "Binary Export Service" })
@versioned(Versions)
namespace Contoso.BinaryExport;

enum Versions {
  @armCommonTypesVersion(Azure.ResourceManager.CommonTypes.Versions.v5)
  `2024-06-01`,
}

model ExportRequest {
  sourceId: string;
}

// LRO: POST returns 202, final result is bytes
@action("export")
op exportBinary is ArmResourceActionAsync<
  // Resource type (for RBAC, etc.)
  ExportResource,
  // Request body type
  ExportRequest,
  // Final response type (bytes)
  {@body data: bytes}
>;

model ExportResource is TrackedResource<{}> {
  ...ResourceNameParameter<ExportResource>;
}

NOTE:

• DO add changelog under https://github.com/Azure/typespec-azure/tree/main/.chronus/changes
• For runtime bahavior, since the response type is bytes, you could refer to https://github.com/microsoft/typespec/tree/main/packages/http-specs/specs/payload/content-negotiation which contains test case whose response type is also bytes.
  </issue_description>

Comments on the Issue (you are @copilot in this section)

@msyyc Context: From issue https://github.com/Azure/autorest.python/issues/3247, I know that https://github.com/microsoft/fabric-rest-api-specs is using swagger and swagger codegen tool. If we hope them to migrate to Typespec, we should support their desired feature.

• Fixes [spector] Add a POST long running API whose response type is bytes #3714

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