Add network-ping CLI command to test container connectivity

[Copilot]

Summary

This PR adds a new network-ping CLI command that allows users to test connectivity between containers in a network from the command line. The feature exposes the existing Ping functionality through a user-friendly CLI interface.

Changes Made

New CLI Command

• Added network-ping command: basic-docker network-ping <network-id> <source-container-id> <target-container-id>
• Updated usage documentation: Added the new command to the help text in printUsage()
• Follows existing patterns: Uses the same error handling and argument validation style as other network commands

Testing

• Added TestNetworkPingCLI test: Comprehensive test covering success and error cases
• All existing tests pass: No regression in functionality (9/9 tests passing)
• Error handling tested: Validates proper error messages for invalid networks and containers

Usage Example

# Create a network and attach containers
./basic-docker network-create demo-network
./basic-docker network-attach net-1 container-a
./basic-docker network-attach net-1 container-b

# Test connectivity between containers
./basic-docker network-ping net-1 container-a container-b
# Output: Pinging from 192.168.1.2 to 192.168.1.3: Success

# Error cases are handled gracefully
./basic-docker network-ping net-1 container-a non-existent
# Output: Error: one or both containers are not in the network

Implementation Details

• Minimal changes: Only 11 lines of new code in main.go plus test coverage
• No breaking changes: Fully backward compatible with existing functionality
• Reuses existing logic: Leverages the already-tested Ping() function from network.go
• Consistent error handling: Follows the same error reporting pattern as other network commands

Validation

✅ All tests pass (including new test case)
✅ CLI command works correctly for success and error scenarios
✅ Help text updated to include new command
✅ Code follows existing style and patterns
✅ No impact on existing functionality

This enhancement makes the networking stack more complete by providing users with a simple way to verify container connectivity through the CLI, complementing the existing network management commands.

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[j143]

Looks good to me.