Fix/cross chain signer removal 295

[rabsqueen]

Summary

Fixes a high-priority logic error in cross_chain_verifier::remove_authorized_signer.

The function previously contained two nearly identical signer-removal blocks. Both blocks checked signer existence, removed the signer from storage, and decremented SignerCount, causing the count to be decremented twice for a single removal and potentially underflowing the u32 counter.

What Changed

🐛 Removed Duplicate Removal Logic

• Removed the redundant signer-removal block.
• Signer existence is now checked once.
• The authorized signer is removed from storage exactly once.
• SignerCount is updated exactly once per successful signer removal.

🔢 Added Underflow Protection

Preserved/enforced the counter guard before decrementing:

if count > 0 {
    count - 1
}

This prevents SignerCount from wrapping when the counter is already zero.

Before

remove_authorized_signer()
        ↓
Check signer exists
        ↓
Remove signer
        ↓
SignerCount -= 1
        ↓
Duplicate block
        ↓
Check signer exists again
        ↓
Remove signer again
        ↓
SignerCount -= 1 ❌

A single signer removal could therefore decrement the counter twice.

After

remove_authorized_signer()
        ↓
Check signer exists
        ↓
Remove signer once
        ↓
if count > 0
        ↓
SignerCount -= 1

The counter now accurately represents the number of authorized signers.

Validation

Verified that:

• Duplicate signer-removal logic has been removed.
• A signer is removed from storage exactly once.
• SignerCount is decremented exactly once per successful removal.
• Counter underflow is prevented.
• Existing signer-existence validation is preserved.
• Removing an authorized signer does not cause SignerCount to wrap.

Acceptance Criteria

• Remove the duplicate removal block.
• Check signer existence once.
• Remove the signer exactly once.
• Decrement SignerCount exactly once.
• Protect against counter underflow with count > 0.
• Preserve existing authorization and storage behaviour.

Result

remove_authorized_signer now performs a single, consistent signer-removal operation and maintains an accurate SignerCount without the risk of u32 underflow or counter corruption.

Closes #295

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[Dev-Zully]

@rabsqueen resolve conflit