refactor(debit): extract shared check helper (#265)

[ALIPHATICHYD]

Summary

Closes #265

This PR extracts repeated inline debit precondition checks and debiting logic in contracts/escrow/src/lib.rs into private helper functions:

• check_debit_precondition: Validates that an agent's prepaid credit balance is sufficient for projected usage bills, panicking with EscrowError::InsufficientCreditBalance (Document the schema-versioning and migration model for redeployments #28) if exceeded.
• debit_agent_credit: Deducts min(billed, credit_balance) from the agent's prepaid balance in storage and publishes the cred_deb event.

Key Changes

• contracts/escrow/src/lib.rs:
  • Route record_usage through check_debit_precondition
  • Route settle through debit_agent_credit
• contracts/escrow/src/test.rs:
  • Added unit tests covering exact credit balance, zero credit balance, partial credit drawdown, and event emission edge cases.
  • Fixed pre-existing clippy warnings (duplicated-attributes and unnecessary-cast).

Verification & Test Output

$ cargo fmt --all -- --check
Passed.

$ cargo clippy --all-targets -- -D warnings
Finished `dev` profile [unoptimized + debuginfo] target(s) in 3.56s

$ cargo test
running 293 tests
test test::test_accept_admin_transfer_clears_pending_slot ... ok
test test::test_accept_admin_transfer_requires_auth ... ok
test test::test_accept_admin_transfer_replaces_admin ... ok
test test::test_record_usage_allows_exact_credit_balance ... ok
test test::test_record_usage_allows_uncredited_agent ... ok
test test::test_record_usage_rejects_insufficient_credit_balance - should panic ... ok
test test::test_settle_partial_credit_balance_drawdown ... ok
...
test result: ok. 293 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 5.12s

[mikewheeleer]

@ALIPHATICHYD solid and well-scoped, merging now 🚢