Improve tests and increase coverage

[Copilot]

This PR addresses feedback from code review to remove unnecessary code and adds comprehensive test coverage for critical components.

Changes

• AgentExecutor.sol: Removed unused deposit() function that was a v1 artifact for agent fee management, no longer needed in v2
• Bitfield.sol: Added validatePadding() to enforce zero-padding in bitfields, preventing malformed bitfield attacks
• BeefyClient.sol: Integrated padding validation in submitInitial(), submitFinal(), and submitFiatShamir() flows
• Test coverage: Added comprehensive tests for Bits, MMRProof, SubstrateMerkleProof, SubstrateTypes; enhanced BeefyClient and Gateway test suites
• Test utilities: Created shared MerkleLib helper to eliminate duplicated Substrate-compatible Merkle tree construction across test files

Security improvements

The bitfield padding validation prevents adversaries from submitting bitfields with malicious padding bits that could bypass signature verification checks:

// Before: No validation - attacker could set padding bits
function submitInitial(Commitment calldata commitment, uint256[] calldata bitfield, ...) {
    // ...
}

// After: Padding bits must be zero
function submitInitial(Commitment calldata commitment, uint256[] calldata bitfield, ...) {
    Bitfield.validatePadding(bitfield, vset.length);
    // ...
}

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• foundry.paradigm.xyz
  • Triggering command: /usr/bin/curl curl -L REDACTED (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

[yrong]

It looks like we need to configure the Copilot agent according to
https://docs.github.com/en/copilot/how-tos/use-copilot-agents/coding-agent/customize-the-agent-firewall,
as it appears to download the Foundry stack for running the contract tests and reviewing this PR.