feat(antd-csharp): gRPC external-signer prepare/finalize parity with REST

[Nic-dorman]

Summary

Replaces 7 NotSupportedException stubs on AntdGrpcClient with real implementations. Consumers can now swap an AntdRestClient for an AntdGrpcClient without code change in the external-signer flow.

Depends on #140 (antd-side UploadService + chunks prepare/finalize).

Methods replaced

method	purpose
PrepareUploadAsync(path, visibility)	external-signer prepare for a file
PrepareUploadPublicAsync(path)	convenience wrapper for visibility="public"
PrepareDataUploadAsync(data, visibility)	external-signer prepare for in-memory bytes
FinalizeUploadAsync(uploadId, txHashes)	finalize wave-batch upload
FinalizeMerkleUploadAsync(uploadId, winnerPoolHash)	finalize merkle-batch upload
PrepareChunkUploadAsync(data)	single-chunk external-signer prepare
FinalizeChunkUploadAsync(uploadId, txHashes)	single-chunk finalize, returns address

Wallet stubs remain — wallet-over-gRPC is a separate ticket.

Implementation notes

• New UploadService.UploadServiceClient _upload field constructed alongside the existing service clients.
• Introduces an internal constructor that accepts pre-built service clients so tests can pass mock subclasses; the public string-target constructor delegates via GrpcChannel.ForAddress.
• MapPrepareResponse helper builds PrepareUploadResult, populating merkle-only fields (Depth, PoolCommitments, MerklePaymentTimestamp) only when PaymentType == "merkle".

Project plumbing

• Antd.Sdk.csproj picks up upload.proto and adds InternalsVisibleTo("Antd.Sdk.Tests") so the internal test constructor is visible to the test project.
• Antd.Sdk.Tests.csproj adds the Grpc.Core.Testing NuGet package — provides TestCalls.AsyncUnaryCall<TResponse>(...), the canonical helper for wrapping a pre-canned Task<TResponse> in the AsyncUnaryCall<TResponse> return type the generated *Async methods produce.

Tests

New Antd.Sdk.Tests/GrpcClientTests.cs:

• MockChunkServiceClient and MockUploadServiceClient subclass the generated service clients and override the relevant virtual *Async methods.
• The other (unused) service clients are constructed with a stub CallInvoker that throws if exercised — keeps the constructor signature happy without spinning a real server.
• 11 xunit cases parallel to the antd-rust / antd-go / antd-py / antd-java / antd-kotlin suites: visibility forwarded/omitted, public convenience wrapper, wave-batch vs merkle, DataMapAddress presence on public, merkle finalize, chunk already-stored short-circuit, chunk finalize body forwarding.

Gates passed on dev2

• dotnet build Antd.Sdk/Antd.Sdk.csproj clean
• dotnet test Antd.Sdk.Tests/Antd.Sdk.Tests.csproj — 43/43 (32 existing REST + 11 new gRPC)