feat: Add two-step ownership transfer and admin events (#80)

[SIDDHANTCOOKIE]

Summary

This PR hardens ownership management and improves observability of admin state changes in the Chainvoice smart contract.

Fixes #80

Problem

• Single-step ownership transfers are error-prone and irreversible
• Ownership changes are not explicitly acknowledged by the recipient
• Fee and treasury updates lacked deterministic on-chain signals for indexers and off-chain services

Solution

• Implemented a two-step ownership transfer mechanism using a pendingOwner state variable
• Ownership transfer requires explicit acceptance by the pending owner, preventing accidental loss of control
• Added the ability for the current owner to cancel an in-progress transfer
• Introduced custom errors for gas-efficient and explicit revert reasons
• Emitted structured events for ownership lifecycle and admin parameter updates

Technical Details

• Ownership lifecycle enforced via explicit state checks rather than implicit modifiers
• Zero address and self-transfers are rejected at initiation
• Custom errors replace string-based reverts to reduce gas usage
• Admin setters now emit events with previous and new values for reliable off-chain indexing
• Pattern aligns with established Ownable2Step semantics while remaining contract-native

Status

• Fully backward compatible with existing integrations
• No changes to external function signatures or observable behavior

I just started contributing to StabilityNexus. Requesting review @Zahnentferner 🙏

Summary by CodeRabbit

• New Features

  • Two-step ownership transfer: initiate, accept, and cancel ownership transfers.
  • Emits events for ownership changes, fee updates, and treasury updates; fee/treasury updates now log previous and new values.
  • Improved validation and error handling for ownership and treasury updates.

• Tests

  • Added comprehensive tests covering ownership flows, validation scenarios, cancellations, and admin updates.

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[coderabbitai]

📝 Walkthrough

Walkthrough

Added a two-step ownership transfer flow (initiate, accept, cancel), new ownership-related errors and events, and updated admin setters to emit events. Tests covering ownership flows and admin event emissions were added.

Changes

Cohort / File(s)	Summary
Ownership Transfer Implementation contracts/src/Chainvoice.sol	Added pendingOwner (public), initiateOwnershipTransfer(address) (onlyOwner), acceptOwnership(), cancelOwnershipTransfer() (onlyOwner). Introduced InvalidNewOwner and OwnershipNotPending errors and OwnershipTransferInitiated, OwnershipTransferred, OwnershipTransferCancelled events.
Admin Events & Validations contracts/src/Chainvoice.sol	setFeeAmount(uint256) now emits FeeUpdated(previousFee, newFee) before change. setTreasuryAddress(address) now validates non-zero and emits TreasuryAddressUpdated(previousTreasury, newTreasury) before change. Added corresponding events.
Tests — Ownership & Admin contracts/test/Chainvoice.t.sol	Added tests covering: initiating transfer, invalid new owner rejection, accepting transfer by pending owner, rejecting non-pending acceptance, canceling transfer, rejecting cancel when none pending, and verifying fee/treasury update events and state.

Sequence Diagram(s)

sequenceDiagram
  participant Owner as Owner (current)
  participant Contract as Chainvoice (contract)
  participant Pending as PendingOwner

  Owner->>Contract: initiateOwnershipTransfer(newOwner)
  activate Contract
  Contract-->>Owner: OwnershipTransferInitiated event
  Contract-->>Pending: pendingOwner set\n(pendingOwner = newOwner)
  deactivate Contract

  Pending->>Contract: acceptOwnership()
  activate Contract
  alt caller == pendingOwner
    Contract-->>Owner: OwnershipTransferred event (previousOwner->newOwner)
    Contract-->>Pending: owner updated, pendingOwner cleared
  else not pending
    Contract-->>Pending: revert OwnershipNotPending()
  end
  deactivate Contract

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Estimated code review effort

🎯 4 (Complex) | ⏱️ ~45 minutes

Poem

🐰 I nudged the keys from paw to paw,

two hops to pass the keeper's law.
Events now sing where changes lie,
Pending waits, then claims the sky. 🥕✨

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: adding two-step ownership transfer and admin events, which matches the core modifications in the PR.
Linked Issues check	✅ Passed	All requirements from issue #80 are met: two-step ownership transfer (initiateOwnershipTransfer, acceptOwnership, cancelOwnershipTransfer), ownership and admin events with previous/new values, custom errors for gas efficiency, and input validation with access control.
Out of Scope Changes check	✅ Passed	All changes are directly related to implementing the two-step ownership transfer pattern and admin events specified in issue #80; no unrelated modifications detected.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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[SIDDHANTCOOKIE]

@kumawatkaran523 pls review when convenient