Stub implementations used in production node startup

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Production node uses stub implementations for critical components.

Location:
https://github.com/decipherhub/cipherbft/blob/main/crates/node/src/node.rs#L779-L781

Stubs used:

• StubMempoolApi - https://github.com/decipherhub/cipherbft/blob/main/crates/rpc/src/adapters.rs#L1155
• StubExecutionApi - https://github.com/decipherhub/cipherbft/blob/main/crates/rpc/src/adapters.rs#L1465
• StubNetworkApi - https://github.com/decipherhub/cipherbft/blob/main/crates/rpc/src/adapters.rs#L1758

These stubs return placeholder values and don't provide real functionality.