fix: add membership verification during add_signature

[kbizikav]

Fix problem imunifi report 56733.
Added verification for pubkey membership besides signature verification.

[Copilot]

Pull Request Overview

This PR adds security validation to the add_signature method by verifying that a public key exists in the proposal memo before accepting its signature. This prevents unauthorized signatures from being added to blocks.

Key Changes:

• Added membership verification for public keys in the signature acceptance flow
• Implemented consistent error handling for unauthorized public key attempts

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
block-builder/src/app/storage/redis_storage.rs	Added pubkey membership check before signature verification in Redis storage implementation
block-builder/src/app/storage/memory_storage.rs	Added pubkey membership check before signature verification in memory storage implementation

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