feat: Escrow Milestone Tracking and State Management

[Cyberking99]

This PR introduces robust internal state management for the escrow contract, supporting fragmented payout structures through multiple sequentially unlockable milestones with specific amounts.

Key Changes

• Enhanced Storage Schema:
  • Introduced Milestone and MilestoneStatus (Pending, Released) structures.
  • Upgraded EscrowJob to use a Vec<Milestone>, allowing for flexible project configurations where each milestone can have a unique payout amount.
• Improved Lifecycle Management:
  • create_job: New entry point to initialize a job in the Setup phase.
  • add_milestone: Allows clients to define specific milestone amounts (flat amounts) during setup.
  • Validated deposit: Ensures the sum of all milestones matches the total deposit exactly before funding the project.
• State Machine Refinement:
  • Meticulously tracked transitions: Setup -> Funded -> WorkInProgress -> Completed (or Disputed).
  • Added WorkInProgress state to better reflect the job's lifecycle after the first milestone release.
• Granular Retrieval:
  • Added a public get_milestone_status function to retrieve the current status of all milestones for any given Job ID.
• Updated Test Suite:
  • Overhauled existing tests to align with the new API.
  • Added test_variable_milestone_amounts to verify complex project structures (e.g., a 20%, 30%, 50% split).

Verification Results

All tests passed successfully (cargo test):

• test_happy_path_lifecycle
• test_variable_milestone_amounts
• test_dispute_50_50_split
• test_refund
• test_exhaustive_release_funds_path
• Security/Edge cases: test_double_init, test_deposit_with_wrong_total_panics, test_unauthorized_release, test_raise_dispute_by_client_locks_funds.

Closes #14

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[Cyberking99]

Kindly review @soomtochukwu