Fix audit hardening issues

[Lchangliang]

Summary

This PR addresses several gravity-audit findings:

• Fixes Galxe/gravity-audit#31 by making BlockBufferManager init validation return errors instead of leaving the manager uninitialized after early return.
• Fixes Galxe/gravity-audit#516 by replacing JWK/DKG deserialization unwraps with guarded handling that drops malformed validator transactions.
• Fixes Galxe/gravity-audit#522 by synchronizing epoch-change readiness with the BlockStateMachine mutex and preventing ordered-block reads/writes during the release-before-consume transition window.
• Fixes Galxe/gravity-audit#598 by adding Prometheus counters for GCEI filtered transactions and coinbase fallback paths.

Validation

• CARGO_TARGET_DIR=/private/tmp/gravity-sdk-check-target RUSTFLAGS="--cfg tokio_unstable" cargo check -p block-buffer-manager
• LC_ALL=en_US.UTF-8 LC_CTYPE=en_US.UTF-8 LANG=en_US.UTF-8 CARGO_TARGET_DIR=/private/tmp/gravity-sdk-check-target RUSTFLAGS="--cfg tokio_unstable" cargo check -p gravity_node