feat(devnet-sdk): add chain descriptor validation

[DeVikingMark]

Description

Replaces TODO comments in chain.go with comprehensive descriptor validation to prevent runtime panics when creating chains from malformed descriptors. Adds validation functions for chain descriptors, node configurations, and L2 chain structures with clear error messages.

Tests

• TestChainFromDescriptorValidation with 9 test cases covering nil descriptors, empty chain IDs, missing nodes, invalid services/endpoints, and port validation
• TestL2ChainFromDescriptorValidation with 3 test cases for L2-specific validation scenarios
• All existing tests continue to pass, ensuring no regression

Additional context

The original code had TODO comments indicating potential panic situations in newChainFromDescriptor() and newL2ChainFromDescriptor() functions. Without proper validation, the code could panic when accessing nested descriptor fields that might be nil or missing. This fix ensures graceful error handling with descriptive error messages while maintaining full backward compatibility.

Metadata

• Fixes Handle incorrect descriptors better in devnet-sdk/system/chain.go #16522