Avoid same peer rebalances

[markettes]

• Mandatory on RebalanceRequest: SourceChannelId is now int and TargetPubkey is now string (both previously nullable). RebalanceService.RebalanceAsync validates each up front with a
  clear ArgumentException.
• gRPC: RequestRebalance rejects requests missing source_channel_id or target_pubkey with StatusCode.InvalidArgument before reaching the service. Proto field comments updated to mark
  both required (the optional wire-protocol modifier stays so existing serialized payloads still parse).
• No-op guard: the service also rejects requests where the receiving peer (TargetPubkey) is the same peer as the source channel's counterparty — that would route the sats straight back
  and accomplish nothing.
• Modal (NewRebalanceModal.razor): "(optional)" label and "let LND choose" placeholder removed; receiving-peer dropdown filters out the source channel's counterparty; CanSubmit requires
  both fields and forbids the no-op combination; if the user changes the source channel and the previously-picked receiving peer becomes invalid, it gets cleared.

The Rebalance model fields are intentionally left nullable in this PR — tightening the DB schema needs a data backfill / cleanup decision that's out of scope here.

[Copilot]

Pull request overview

This PR makes rebalance requests stricter and prevents “no-op” rebalances by requiring SourceChannelId and TargetPubkey end-to-end (UI → gRPC → domain), and adding a guard to reject selecting the source channel’s counterparty as the receiving peer.

Changes:

• Make RebalanceRequest.SourceChannelId and RebalanceRequest.TargetPubkey mandatory and validate them early in RebalanceService.RebalanceAsync.
• Add gRPC-layer InvalidArgument guards for missing/invalid source_channel_id and target_pubkey before the node lookup.
• Update the rebalance modal to require a receiving peer and filter out the source channel counterparty; expand/adjust test coverage accordingly.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
test/NodeGuard.Tests/Services/RebalanceServiceTests.cs	Updates existing tests for new required request fields and adds new validation/no-op-guard tests.
test/NodeGuard.Tests/Rpc/NodeGuardServiceTests.cs	Adds gRPC validation tests for missing required fields and updates existing cases to reach intended code paths.
src/Shared/NewRebalanceModal.razor	Makes receiving peer required, filters out counterparty peer, and enforces submission constraints in the UI.
src/Services/RebalanceService.cs	Changes RebalanceRequest to non-nullable fields and adds validation + no-op counterparty guard in RebalanceAsync.
src/Rpc/NodeGuardService.cs	Rejects missing/invalid source_channel_id and target_pubkey with InvalidArgument before calling the domain service.
src/Proto/nodeguard.proto	Updates field comments to mark source_channel_id and target_pubkey as required (keeping wire optional).

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