feat(SC-ESC-020): add happy-path integration tests, getters, and chec…

[extolkom]

closes #374

Summary
Implements comprehensive happy-path integration tests for the escrow smart contract as part of the Lance marketplace ecosystem.
Changes

Fixed release_milestone panic message to "only client can release" for proper auth error signaling
Added get_escrow_balance getter to query remaining funds from persistent storage
Added get_milestone getter to query individual milestone parameters
Rewrote reentrancy guard tests to be deterministic and race-condition free
Added test_get_escrow_balance_decreases_on_release integration test
Added test_multiple_jobs_isolated integration test
Audited and completed all checked arithmetic using checked_add and checked_mul

Requirements Checklist

Storage keys and traits in contracts/escrow/src/lib.rs analyzed
Checked arithmetic (checked_add, checked_mul) applied throughout
env.require_auth enforced on all state-modifying actions
Public getter functions exposed for escrow parameters and balances

Acceptance Criteria

Contract compiles to WASM target
All test suites pass with zero gas-limit violations
Unauthorized state changes yield auth erro

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