Enable Mainnet wallet helpers and remove Testnet self-link

[JOY (JOY)]

Motivation

Mainnet explicitly disabled the frontend web3 wallet helper while Testnet inherited the supported wallet list. Testnet also advertised itself as an external explorer, which produced a redundant self-link.

This change aligns both DOS Chain explorers with the supported Blockscout frontend configuration and adds deployment guards so these regressions are caught before and after rollout.

Changelog

Enhancements

• Let Mainnet inherit the supported metamask, rabby, coinbase, and token_pocket wallet helpers.
• Verify the effective wallet list for both Mainnet and Testnet during deployment.

Bug Fixes

• Remove the Testnet self-reference from NEXT_PUBLIC_NETWORK_EXPLORERS.
• Reject a future Testnet self-link in both local and public deployment verification.

Incompatible Changes

• None.

Upgrading

• No database or API migration is required.
• The configuration deployment restarts only the frontend services through the existing workflow.

External monitoring

GCP Cloud Monitoring now checks the Mainnet metadata proxy, Testnet metadata proxy, and direct Blockscout metadata upstream every 15 minutes from three regions. The alert policy notifies the existing JOY AI email channel when at least two regions fail, and distinguishes an upstream outage from a DOS proxy failure. This monitoring configuration is external to this repo diff.

Validation

• git diff --check
• python scripts/validate-blockscout-env-parity.py
• Docker Compose effective configuration for Mainnet and Testnet
• actionlint for .github/workflows/deploy-config.yml
• Live metadata proxy responses contain a valid addresses object
• Independent review by two agents: APPROVE, no must-fix findings

Checklist

• I have performed a self-review of my changes
• I have added deployment guards for the changed configuration
• I have verified no database rollback path was introduced
• The change is compatible with DOS Chain as an Avalanche L1

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