feat: implement PingNode gRPC handler

[roganlynch]

Implements the PingNode gRPC handler.

Changes

• PingNode RPC handler: Server-side implementation of the PingNode gRPC method
• Request validation: Validates node identification and ping parameters
• PingManager integration: Coordinates with the PingManager to execute pings
• Response handling: Returns ping results through the gRPC response

Implementation Details

The handler receives PingNode requests through gRPC, validates the request parameters, invokes the PingManager to execute the ping operation, and returns the results. This provides the control plane API for triggering on-demand node pings.

Context

Part of the work discussed in #2902 to add ping functionality to headscale.

Dependencies

• Requires PR #2941 (protobuf definitions)

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Note: This code was generated with assistance from Claude Sonnet 4.5 via cline.bot