feat(escrow): implement partial milestone payment release and tracking

[kosisochukwu1234]

Closes #190

Implement secure partial payment releases for milestone-based contracts. This PR allows clients to approve and release funds for individual milestones while ensuring escrow balance integrity and preventing double spending.

Changes Made:

• Balance Tracking: Added EscrowBalance to DataKey to track the remaining escrow balance. The state is incremented correctly upon funding and decremented on release (as well as on refunds and disputes).
• Client-only Authorization: Modified the release function so that only the client can trigger milestone releases.
• Idempotency checks: Explicitly verify that the milestone is not already in the Released state before processing the payment, preventing duplicate payouts.
• Event Emission: Ensure events properly log balance updates (balance event) following fund operations, refunds, and milestone releases.
• Test Coverage Updated: Updated existing tests (e.g. test_happy_path, test_unauthorized_release_fails) to reflect the new authorization model and verified that the escrow balance logic is successfully checked alongside the release.

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