Handle Zero-Balance Edge Case on Settlement

[Luluameh]

Source: MORE_BACKEND_ISSUES.md (Issue 19)

Background
If a session runs completely out of escrowed funds, it should auto-close, but rounding errors could leave "dust".
Goal
Implement a cleanup routine that zeroes out dust balances and finalizes the session gracefully.
Acceptance Criteria

• Any remaining balance < 10 stroops should be sent to the expert or platform fee pool.
• Session status set to Completed.
  Key Files
  contracts/src/lib.rs

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