Add Base L2 remote chain support

[dominicletz]

Summary

• Add Chains.Base with registry and developer fleet addresses, and register it in RemoteChain production chain list.
• Derive Contract.NodeRegistry chain ID from DiodeClient.Shell.Moonbeam instead of a hardcoded constant, with commented post-freeze Base migration addresses.
• Bump diode_client to pick up DiodeClient.Shell.Base support and add regression tests for Base chain metadata.

Test plan

• DIODE_MINIMAL_TEST=1 mix test --no-start test/chains/base_test.exs
• mix compile on a clean checkout
• Verify Base RPC/WS endpoints resolve via RemoteChain.ChainList in a running node

[gemini-code-assist]

Code Review

This pull request adds support for the Base L2 chain by introducing the Chains.Base module, updating the remote chains list, and adding corresponding tests. It also refactors Contract.NodeRegistry to dynamically fetch the Moonbeam chain ID. The reviewer noted that a commented-out line in node_registry.ex uses an incorrect module reference (Shell.Base instead of DiodeClient.Shell.Base), which would cause compilation issues if uncommented.

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