Fix unguarded /outspends response parsing in MempoolSpaceIndexerApi

[dangershony]

Problem

FetchUtxoAsync and GetIsSpentOutputsOnTransactionAsync in MempoolSpaceIndexerApi.cs called the indexer's /outspends endpoint and immediately deserialized the response as JSON without checking IsSuccessStatusCode first — unlike every other call site in this file, which all guard with the standard pattern:

_networkService.CheckAndHandleError(response);
if (!response.IsSuccessStatusCode)
    throw new InvalidOperationException($"Indexer {IndexerHost(client)} returned an error: {response.ReasonPhrase}");

When the indexer returns a plain-text error body (e.g. Internal Server Error) instead of JSON — which happens under load — this crashed with a confusing:

System.Text.Json.JsonException: 'I' is an invalid start of a value. Path: $ | LineNumber: 0 | BytePositionInLine: 0.

...instead of a clear, actionable indexer error.

How this was found

While chasing an intermittent UAT failure in SendFundsTest (three wallets sending funds to each other in rounds against the Angornet test indexer), SendAmount failed with the JSON parse exception above while test.indexer.angor.io was under load.

Fix

Added the same IsSuccessStatusCode guard (and _networkService.CheckAndHandleError) before deserializing in both call sites, matching the existing pattern used everywhere else in this file.

After the fix, the same underlying indexer issue now surfaces as a clear, diagnosable error instead of a crash:

Indexer test.indexer.angor.io returned an error: Internal Server Error

This doesn't fix indexer instability under load (that's a separate, ongoing infra concern — see #971), but it ensures failures are always surfaced with a meaningful message instead of being masked by an unrelated JSON parse exception.

Testing

• dotnet test src/shared/Angor.Shared.Tests/Angor.Shared.Tests.csproj — 160/160 passed.
• Re-ran App.Test.Uat/SendFundsTest before/after: confirmed the JSON crash is gone and replaced with the clear indexer error message. The test itself still intermittently fails due to indexer load, tracked separately.