rpc: improve ConnectPeer error message when no host provided

[TechLateef]

Summary

Fixes #10617

When ConnectPeer is called with only a pubkey and no host address, it now returns a descriptive error message "no addresses known for peer <pubkey>" instead of a generic EOF error.

Changes

• Added host validation check in ConnectPeer RPC method
• Returns proper gRPC InvalidArgument status with descriptive message
• Added unit test to verify the new error behavior

Testing

• Added TestConnectPeerNoHost test case
• All existing tests continue to pass
• Verified error message format follows LND conventions

This helps clients distinguish this failure case and guide users appropriately (e.g., "try providing host:port directly").

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the user experience for the ConnectPeer RPC by providing a more informative error message when a connection attempt is made without specifying a host address. Previously, such attempts resulted in a vague EOF error, making it difficult for clients to diagnose the issue. The change introduces explicit validation and returns a clear InvalidArgument gRPC status, guiding users to provide the necessary host information.

Highlights

• Improved Error Handling: The ConnectPeer RPC method now returns a descriptive InvalidArgument gRPC error when a host address is not provided, replacing a generic EOF error.
• Host Validation: A new validation check was introduced in the ConnectPeer method to explicitly detect and handle cases where the Addr.Host field is empty.
• New Unit Test: A dedicated unit test, TestConnectPeerNoHost, was added to ensure the correct error message and gRPC status are returned when a peer connection is attempted without a host.

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[gemini-code-assist]

Code Review

This pull request introduces a new validation check in the ConnectPeer RPC endpoint to ensure that a host address is provided. If the host is empty, an InvalidArgument gRPC error is returned with a descriptive message. A new unit test, TestConnectPeerNoHost, has been added to verify this error handling. The reviewer suggests using status.Errorf directly for improved readability and idiomatic Go when constructing the gRPC status error.

[lightninglabs-deploy]

@TechLateef, remember to re-request review from reviewers when ready

[TechLateef]

@saubyk Could you please take a look at this when you have a chance?

this implements:
host validation check in ConnectPeer RPC method
Returns proper gRPC InvalidArgument status with descriptive message
Added unit test to verify the new error behavior

addresses issue #10617

Thanks!