.env.mainnet and .env.sepolia ship a shared default BASE_NODE_L2_ENGINE_AUTH_RAW with no fail-fast guard #1159

Both .env.mainnet and .env.sepolia commit the same hardcoded value for BASE_NODE_L2_ENGINE_AUTH_RAW — byte-identical across the two files and present since the env files were first added. This is a secret that is meant to be unique per deployment, but it ships as a working default. Current behaviour: consensus-entrypoint and execution-entrypoint check only that the variable is non-empty, then use it as-is; because the committed default is non-empty, a deployment that never overrides it starts successfully and runs on the shared default with no warning or error. A safe default would ship a placeholder rather than a usable value, and the entrypoints would fail fast when the placeholder is still in place, so that running on an unconfigured secret is impossible rather than silent. Affected files: .env.mainnet, .env.sepolia, consensus-entrypoint, execution-entrypoint.