Bump MCPG to v0.3.26 and AWF Firewall to v0.27.4

## Summary

Bump two infrastructure dependencies:

| Component | Current | Target | Release |
|---|---|---|---|
| MCP Gateway (mcpg) | `v0.3.25` | `v0.3.26` | https://github.com/github/gh-aw-mcpg/releases/tag/v0.3.26 |
| AWF Firewall | `v0.27.2` | `v0.27.4` | https://github.com/github/gh-aw-firewall/releases/tag/v0.27.4 |

## Changes needed

Update `pkg/constants/version_constants.go`:

```go
const DefaultFirewallVersion Version = "v0.27.4"
const DefaultMCPGatewayVersion Version = "v0.3.26"
```

## Checklist

- [ ] Update `DefaultMCPGatewayVersion` in `pkg/constants/version_constants.go`
- [ ] Update `DefaultFirewallVersion` in `pkg/constants/version_constants.go`
- [ ] Run `make recompile` to update lock files referencing the container images
- [ ] Verify smoke tests pass with new versions
- [ ] Check for any breaking changes in release notes that require codemod or config updates