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

[Nic-dorman]

Summary

Replaces 7 UnsupportedOperationException stubs on AntdGrpcClient with real implementations. Consumers can now swap an AntdRestClient for an AntdGrpcClient without code change in the external-signer flow — both classes already satisfy the IAntdClient interface contract.

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

Methods replaced

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)	finalize merkle-batch upload
prepareChunkUpload(data)	single-chunk external-signer prepare
finalizeChunkUpload(uploadId, txHashes)	single-chunk finalize, returns address

All are suspend fun per the existing interface. Wallet stubs remain — wallet-over-gRPC is a separate ticket.

Implementation notes

• New UploadServiceGrpcKt.UploadServiceCoroutineStub uploadStub alongside the existing service stubs.
• Introduces an internal channel-injecting constructor so tests can pass an in-process ManagedChannel; the public String-target constructor delegates to it.
• mapPrepareUploadResponse helper builds PrepareUploadResult, populating merkle-only fields (depth, poolCommitments, merklePaymentTimestamp) only when paymentType == "merkle".

Proto/build

• Gradle's protobuf plugin auto-generates the new Upload.*, Common.PaymentEntry, and Chunks.PrepareChunk* / Chunks.FinalizeChunk* Kotlin DSL builders + coroutine stubs from ../../antd/proto/antd/v1/.
• lib/build.gradle.kts adds two testImplementation deps — io.grpc:grpc-inprocess and io.grpc:grpc-testing — to support the new in-process server tests.

Tests

New lib/src/test/kotlin/com/autonomi/sdk/GrpcClientTest.kt:

• In-process gRPC server using InProcessServerBuilder / InProcessChannelBuilder.
• 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.
• 11 new tests parallel to the antd-rust / antd-go / antd-py / antd-java suites.

Gates passed on dev2

• ./gradlew :lib:compileKotlin clean
• ./gradlew :lib:test — 47/47 (29 REST + 7 smoke + 11 new gRPC)
• ./gradlew :lib:build — full project passes