feat: Add IAsyncEnumerable support for auto-pagination

[Copilot]

Adds C# 8.0 async stream support for paginated endpoints, eliminating manual while (next != null) boilerplate. Network calls trigger lazily as enumeration advances.

Changes

• New PaginatedStreamExtensions.cs: Extension methods on IUpApi returning IAsyncEnumerable<T> for all paginated endpoints

  • GetTransactionsStreamAsync() - all transactions with filters
  • GetTransactionsStreamAsync(accountId) - account-specific
  • GetAccountsStreamAsync(), GetTagsStreamAsync(), GetAttachmentsStreamAsync(), GetWebhooksStreamAsync(), GetWebhookLogsStreamAsync()
  • All methods accept CancellationToken via [EnumeratorCancellation]

• Updated README: Streaming examples and API reference

• Updated Sandbox: Demonstrations including cancellation and short-circuit patterns

Usage

// Automatic pagination
await foreach (var tx in upApi.GetTransactionsStreamAsync(tag: "food")) 
{
    Process(tx); // Fetches next page when buffer exhausted
}

// Short-circuits network calls
await foreach (var tx in upApi.GetTransactionsStreamAsync())
{
    if (tx.Attributes.Amount.Value > 100) break;
}

// LINQ composition (with System.Linq.Async)
var expensive = await upApi.GetTransactionsStreamAsync()
    .Where(t => t.Attributes.Amount.Value < -100)
    .FirstOrDefaultAsync();

Zero breaking changes. Maintains AOT compatibility.

Original prompt

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This section details on the original issue you should resolve

<issue_title>feat: Add IAsyncEnumerable support for auto-pagination</issue_title>
<issue_description>Description:

Context
Current manual while (next != null) pagination is imperative boilerplate. Consumers frequently need to stream result sets (ETL, accumulation) without managing page cursors.

Proposal
Expose IAsyncEnumerable<T> wrappers for paginated endpoints to enable C# 8.0 Async Streams.

Spec

• Behavior: Lazy evaluation. Network calls trigger only as enumeration advances past the current page buffer.
• Cancellation: Pass CancellationToken through the enumerator.

Usage Definition

// Streaming (Automatic paging behind the scenes)
await foreach (var tx in upApi.GetTransactionsStreamAsync(tag: "food")) 
{
    // Yields items individually; blocks on network only when page buffer is exhausted
    Process(tx);
}

// Composition (System.Linq.Async)
var targetTx = await upApi.GetTransactionsStreamAsync()
    .Where(t => t.Attributes.Amount.Value > 100)
    .FirstOrDefaultAsync(); // Short-circuits network calls on match

Implementation Note
Likely requires an extension method or core refactor to yield items from Data array, then await Next link, rinse/repeat.</issue_description>

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

• Fixes feat: Add IAsyncEnumerable support for auto-pagination #5

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