Handle incorrect descriptors better in devnet-sdk/system/chain.go

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

## Problem

The code in `devnet-sdk/system/chain.go` has TODO comments indicating potential panic situations:

```go
// TODO: handle incorrect descriptors better. We could panic here.
```

These TODOs exist in:
- `newChainFromDescriptor()` function (line 198)
- `newL2ChainFromDescriptor()` function (line 223)

## Impact

- Code can panic at runtime if descriptors are malformed
- No validation for required fields like chain ID, nodes, services, endpoints
- Poor developer experience with cryptic error messages

## Expected Solution

- Add proper validation for chain descriptors
- Replace TODO comments with error handling
- Prevent runtime panics with clear error messages
- Maintain backward compatibility

## Files Affected

- `devnet-sdk/system/chain.go`
- `devnet-sdk/system/chain_test.go` (for tests)

Start in devnet-sdk/system/chain.go by reading newChainFromDescriptor() and newL2ChainFromDescriptor(), including the TODO locations. Then inspect devnet-sdk/system/chain_test.go and run its tests. Done means malformed descriptors return clear errors instead of causing panics, while valid descriptors continue to work.

Written by the indexing model from the issue text.