fix(client): accumulate RPC extension hooks instead of replacing

[niran]

Summary

• Introduces BaseBuilder wrapper that accumulates extend_rpc_modules and on_node_started hooks in vectors instead of replacing them
• Updates all extensions (TxPoolExtension, MeteringExtension, FlashblocksExtension) to use new add_rpc_module() and add_node_started_hook() methods
• Adds integration test verifying multiple RPC extensions are accessible simultaneously

Problem

Reth's extend_rpc_modules and on_node_started use a replacement model—each call overwrites the previous hook. This caused only the last extension's hooks to execute, making endpoints like base_meterBundle and base_transactionStatus unavailable even when their respective extensions were enabled.

Solution

The new BaseBuilder wrapper collects all hooks during extension setup, then applies them all in a single reth call when launching. This maintains the existing extension API while fixing the accumulation issue.

Closes #438

Test plan

• Verify cargo test --package base-client-node --features test-utils passes
• Confirm all RPC endpoints (base_transactionStatus, base_meterBundle, flashblocks) are accessible when multiple extensions are enabled

[cb-heimdall]

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