Add indexer retry/backoff + concurrency cap; ease SendFundsTest load

[dangershony]

Problem

Chasing intermittent SendFundsTest and MultiFundClaimAndRecoverTest UAT failures against the shared Angornet test indexer (test.indexer.angor.io), which has no healthy fallback (signet.angor.online/signet2.angor.online are both currently unreachable) and returns transient Internal Server Error responses under concurrent load.

Root cause

Wallet balance/UTXO refresh (WalletOperations.UpdateDataForExistingAddressesAsync) fans out one indexer request per address via Task.WhenAll with no bound. For a wallet with many derived addresses — and especially with multiple wallets refreshing concurrently (3 processes in SendFundsTest, 4+ in MultiFundClaimAndRecoverTest) — this fires a burst of simultaneous requests the indexer can't reliably handle.

Unlike PublishTransactionAsync (which already retries broadcasts 3x with backoff), the read paths (FetchUtxoAsync, GetTransactionInfoByIdAsync) had no retry at all, so a single flaky response aborted the whole operation — surfacing as SendAmount failed ... Indexer test.indexer.angor.io returned an error: Internal Server Error or We couldn't load the claimable funds for this project: Internal Server Error.

(This builds on #973, which fixed a related but distinct bug where the same failure mode crashed with a confusing JSON parse exception instead of a clear error message.)

Changes

• MempoolSpaceIndexerApi: add 3-attempt retry with backoff around FetchUtxoAsync and GetTransactionInfoByIdAsync (both pure reads, safe to retry), matching the existing broadcast-retry pattern in PublishTransactionAsync.
• WalletOperations: add a SemaphoreSlim-based throttle (max 8 concurrent) around FetchUtxoForAddressAsync so gap-limit scans and multi-wallet activity don't hammer the indexer with unbounded concurrent requests.
• SendFundsTest:
  • Stagger address-fetch/send calls across the 3 test wallets instead of firing them in a tight simultaneous burst.
  • Add a bounded retry (3 attempts, 5s backoff) to the test's own GetAddress helper — a pure read, safe to retry.
  • Reduce TotalRounds from 10 to 5. The test was firing 43 send transactions total (10×3 main rounds + 4×3 burst rounds + 1 sweep) against a single shared, fragile test indexer — excessive load for a UAT test. 5 rounds still exercises the same code paths (multi-round send/receive, burst/unconfirmed spends, final sweep) with roughly half the indexer load.

Testing

• dotnet test src/shared/Angor.Shared.Tests/Angor.Shared.Tests.csproj — 160/160 passing throughout.
• Re-ran SendFundsTest and MultiFundClaimAndRecoverTest multiple times after each incremental change — both now pass reliably (previously failed intermittently with indexer errors/timeouts at various points: receive-address timeout, send failure, claimable-funds load failure).