Add Pool Close Function After Disbursement (#16)

[vestor-dev]

Add Pool Close Function After Disbursement (#16)

Summary

Implements an admin/creator function to mark pools as closed after full disbursement or cancellation, with optional storage cleanup for resource efficiency on Stellar.

closes #16

Changes

Core Implementation

• New Pool State: Added Closed state to PoolState enum
• New Functions:
  • close_pool(pool_id, caller): Marks a pool as closed (admin-only)
  • is_closed(pool_id): View function to check if a pool is closed
• New Event: pool_closed event emitted when a pool is closed
• New Errors:
  • PoolAlreadyClosed: Prevents closing an already closed pool
  • PoolNotDisbursedOrRefunded: Ensures pools can only be closed after disbursement or cancellation

Business Logic

• Pools can only be closed when in Disbursed or Cancelled state
• Only admin can close pools (authorization required)
• Closing is optional and non-forced - no automatic cleanup
• Once closed, pools cannot be reopened

Testing

Added comprehensive test suite (close_pool_test.rs) with 16 tests covering:

• ✅ Successful closure after disbursement
• ✅ Successful closure after cancellation
• ✅ Prevention of double-closing
• ✅ State transition validation
• ✅ Authorization checks
• ✅ Error handling for invalid states
• ✅ View function behavior across all states
• ✅ Event emission verification
• ✅ Multiple pool scenarios

[martinvibes]

Nice Implementation @vestor-dev
LGTM!