refactor(test): split escrow tests into focused modules

[oderahub]

Summary

Refactored the escrow test suite into focused feature-based modules to
improve maintainability, reviewability, and long-term scalability without
changing contract behavior.

Closes #23

What changed

• split the previous monolithic escrow/src/test.rs into logical modules under
  escrow/src/test/
• kept shared helpers in the root test module to reduce duplication and keep
  setup consistent
• folded the old standalone funding-target and token-integration tests into
  the new module structure
• removed extra #[cfg(test)] module declarations from escrow/src/lib.rs
• updated README.md with test organization guidance and structure notes

Test organization

Tests are now grouped by feature area:

• init.rs — initialization, invoice-id validation, getters, and init
  baselines
• funding.rs — funding, contribution accounting, snapshots, and tier
  selection
• settlement.rs — settlement, withdrawal, investor claims, maturity
  boundaries, and dust sweep
• admin.rs — admin-governed updates, legal hold, migration guards, and
  collateral metadata
• integration.rs — token-wrapper and metadata-only integration assumptions
• properties.rs — proptest-based invariants

Shared helpers remain in escrow/src/test.rs. Each test creates a fresh Env
and local setup so modules do not depend on hidden cross-test state.

Validation

Ran locally:

cargo fmt --all -- --check
cargo test

Result:

• cargo fmt --all -- --check passed
• cargo test passed
• 88 tests passed, 0 failed

Security notes

• no contract logic was changed
• no execution paths or on-chain behavior were modified
• this refactor only reorganizes tests and related documentation
• test isolation was preserved by keeping each test self-contained with fresh
  environment setup
• module wiring and imports were validated by running the full suite after
  the split

Why this helps

This makes the escrow suite easier to audit, extend, and review as coverage
grows, especially around security-sensitive flows like funding, settlement,
claims, admin controls, and boundary conditions.

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[oderahub]

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