test(invoice-token): add integration test for burn-on-settlement callback

[Democodes-flash]

Overview

Adds a cross-contract integration test suite for the invoice-token burn-on-settlement callback, verifying the escrow -> token settlement flow end to end.

Problem

The invoice-token burn / burn_from paths invoked from the settlement escrow were only covered by unit tests in isolation. There was no integration-level coverage proving the cross-contract callback wiring, event emission, and state transitions behave correctly when a settlement escrow triggers the burn.

Proposed Solution

• Introduce a MockSettlementEscrow contract plus a shared Ctx/setup() harness in integration_test.rs
• Cover the happy-path burn_on_settlement and burn_from_on_settlement flows
• Cover failure paths: insufficient balance, invalid amounts, insufficient allowance, expired allowance, paused, and uninitialized contracts
• Verify post-settlement transfer unlock and state persistence across partial and full burns
• Assert emitted events, balances, and allowance state

Acceptance Criteria

• burn_on_settlement happy path burns the correct amount and emits the event
• burn_from_on_settlement burns on behalf of the escrow with a valid allowance
• Insufficient balance and invalid amounts are rejected with the correct errors
• Insufficient and expired allowances are rejected with the correct errors
• Paused and uninitialized contracts reject the burn
• After full settlement, transfers are unlocked and token can move
• State persists correctly across partial and full burn scenarios
• All 108 invoice-token tests pass (97 pre-existing + 11 new); cargo fmt and cargo clippy --all-targets -- -D warnings clean

Closes #158

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