feat(`forge script`): add `--gas-buffer` flag to support fixed overhead for Cosmos-integrated EVM chains

### Component

Forge

### Describe the feature you would like

Hi team 👋

I’d like to propose adding a `--gas-buffer` flag to `forge script` to support cases where a fixed amount of gas needs to be added on top of the estimated gas limit.

This is particularly useful for Cosmos-integrated EVM chains (e.g. Evmos, Canto, Cronos, Initia's Minievm etc.), where there’s often a **fixed gas overhead** (e.g. 10,000~30,000 gas) added by the Cosmos layer that isn’t visible to EVM-based gas estimation.

Even with a high `--gas-estimate-multiplier`, transactions can still fail if the base EVM gas usage is low — because the final gas limit doesn't account for the Cosmos-side fixed fee.

### Suggested flag:
```bash
--gas-buffer
```

### Behavior:
```
finalGas = estimatedGas * multiplier + gasBuffer
```

### Benefits:
- Makes Foundry more robust for Cosmos-EVM chains
- Prevents under-gas failures due to fixed gas layers outside the EVM
- Keeps the scripting workflow clean without needing external estimation logic

Thanks for all your work on Foundry — happy to help test or contribute if this feature moves forward!

### Additional context

_No response_