fix(testnet): target canonical blockchain RPC

[JOY (JOY)]

Summary

• route Testnet HTTP, WebSocket, backend, and user-ops RPC traffic to the canonical JASJZyVT blockchain
• identify the public RPC origin as dos-testnet-r0-JASJZyVT
• add a regression validator that rejects the retired 2EhCz blockchain target
• update the architecture source of truth

Validation

• 29 Python tests passed
• Testnet BENS validator passed
• Blockscout environment parity passed
• Docker Compose config passed
• pinned Caddy image validation passed
• actionlint passed
• git diff --check passed

[chatgpt-codex-connector]

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[gemini-code-assist]

Code Review

This pull request updates the Testnet blockchain ID from the retired hash to the canonical hash across multiple configuration files, environment variables, documentation, and validation scripts. It also introduces automated validation checks and corresponding unit tests to ensure all runtime RPC targets consistently use the canonical blockchain. The feedback suggests referencing the blockchain hash constants directly from the imported module in the test file to eliminate duplication and prevent future desync issues.

[gemini-code-assist]

The blockchain hash constants canonical_blockchain and retired_blockchain are duplicated here from scripts/validate-testnet-bens.py. Since the test class dynamically imports the validation script and stores it in self.module, you can reference these constants directly from self.module to avoid duplication and prevent future desync issues when the blockchain targets are updated.

        canonical_blockchain = self.module.CANONICAL_TESTNET_BLOCKCHAIN\n        retired_blockchain = self.module.RETIRED_TESTNET_BLOCKCHAIN