add export job dotnet sample (pending next dts dp release for multi taskhubs support)

[YunchuWang]

Add the Export History Web App sample for the .NET Durable Task SDK. This is an ASP.NET Core web app that exposes a REST API for creating and managing Durable Task export history jobs, using Durable Task Scheduler (Azure Managed) and Azure Blob Storage.

Does this introduce a breaking change?

No
Pull Request Type

sample

Please describe:
How to Test

Get the code:

Test the code:

Set the required environment variables (or update launchSettings.json):
DURABLE_TASK_CONNECTION_STRING — Durable Task Scheduler connection string
EXPORT_HISTORY_STORAGE_CONNECTION_STRING — Azure Storage connection string
EXPORT_HISTORY_CONTAINER_NAME — Blob container name (e.g. export-history)
Run the sample:
Use the included ExportHistoryWebApp.http file (with the VS Code REST Client extension) or curl to interact with the API at http://localhost:5009.
What to Check

Verify that the following are valid:

The project restores and builds successfully with dotnet build
The app starts and listens on http://localhost:5009
POST /export-jobs creates a new batch or continuous export job
GET /export-jobs/{id} retrieves an export job by ID
GET /export-jobs/list lists all export jobs (with optional query filters)
DELETE /export-jobs/{id} deletes an export job
The README accurately describes the sample, configuration, and API endpoints
The sample follows the same structure and conventions as other .NET samples in dotnet
Note: This sample depends on the Microsoft.DurableTask.ExportHistory NuGet package, which is pending publication (see microsoft/durabletask-dotnet PR).

Testing:

Local running all operations in sample pass against a dts scheduler with single taskhub.
there is a pending issue multi taskhubs export are not supported. pending next dts dp release.

https://msazure.visualstudio.com/One/_git/AAPT-DTMB/pullrequest/14904498

[greenie-msft]

@YunchuWang @torosent I think we should more clearly explain what this feature does and when it’s appropriate to use it. It took me some time to fully grasp what this sample is for.
At a high level, this feature moves orchestration data out of DTS and into a storage account of the customer's choosing. That unlocks several important benefits, such as longer data retention and reduced DTS storage costs, which would be helpful to call out explicitly.

[torosent]

Yes. we need a doc for it