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

[Nic-dorman]

Summary

Adds 7 new methods on GrpcAntdClient mirroring the existing REST AntdClient external-signer surface. Consumers can swap an AntdClient for a GrpcAntdClient without code change in the prepare/finalize flow.

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

Methods

method	purpose
prepareUpload(path, visibility:)	external-signer prepare for a file
prepareUploadPublic(path)	convenience wrapper for visibility: 'public'
prepareDataUpload(data, visibility:)	external-signer prepare for in-memory bytes
finalizeUpload(uploadId, txHashes)	finalize wave-batch upload
finalizeMerkleUpload(uploadId, winnerPoolHash, storeDataMap:)	finalize merkle-batch upload
prepareChunkUpload(data)	single-chunk external-signer prepare
finalizeChunkUpload(uploadId, txHashes)	single-chunk finalize, returns address

Proto regen

./tool/generate_proto.sh now includes antd/v1/upload.proto. Per the existing reference_dart_protoc_plugin_pin convention, protoc_plugin must be pinned to 22.3.0; the regen script's header now states that explicitly.

• New upload.pb.dart / upload.pbgrpc.dart / upload.pbjson.dart.
• chunks.* gain PrepareChunk / FinalizeChunk; common.* adds PaymentEntry.

Implementation notes

• New _uploadStub field constructed alongside the existing service clients.
• Helper _mapPrepareUploadResponse translates the proto into PrepareUploadResult, populating the merkle-only fields (depth, poolCommitments, merklePaymentTimestamp) only when paymentType == "merkle".

Tests

New test/grpc_client_external_signer_test.dart:

• Spins up a real grpc-dart server bound to 127.0.0.1:0 with mock service implementations, then exercises the real GrpcAntdClient against it. This actually tests the proto wire-shape mapping, in contrast to the existing grpc_client_test.dart which uses a hand-rolled fake that doesn't depend on the generated stubs.
• _MockChunkService covers prepareChunk (new + already-stored short-circuit) and finalizeChunk (echoes upload_id).
• _MockUploadService covers prepareFileUpload / prepareDataUpload / finalizeUpload with visibility round-trip via upload_id encoding and merkle vs wave-batch branches.
• 12 new tests parallel to the antd-rust / antd-go / antd-py / antd-java / antd-kotlin / antd-csharp / antd-ruby suites.

Gates passed on dev2

• dart analyze lib/ — 1 pre-existing warning (_doHead unused); no new issues from this PR.
• dart test — 63/63 passing (51 existing + 12 new).