arch: add pre-flight database and network integrity checks to reth-entrypoint

[rdin777]

This PR introduces a robust pre-flight check mechanism to the reth-entrypoint script to improve node resilience and prevent common failure modes in production environments.

Key Architectural Improvements:

Automated DB Integrity Validation:

Adds a call to reth db stats before the main process execution.

Value: Prevents "Crash Loops" caused by corrupted PebbleDB/MDBX states, ensuring the node only starts with a healthy local database.

Flashblocks Connectivity Guard:

Implements a non-blocking connectivity check for the RETH_FB_WEBSOCKET_URL.

Value: Provides early warning in logs if the Flashblocks service is unreachable, preventing silent synchronization delays.

Defensive Boot Sequence:

Ensures that environment variables for authentication and network are validated before exec.

Why this is necessary: Based on recent optimizations in the OP Stack P2P layer and Arbitrum state validation, maintaining high availability (HA) requires proactive checks at the entrypoint level rather than relying solely on the binary's internal error handling.

[cb-heimdall]

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