test: mutation testing for order/trade logic modules

[mostronatorcoder]

Context

Follow-up from #618. Implement mutation testing coverage for the order and trade logic modules — the most critical area in Mostro since it handles real money.

Scope

Target these modules in src/app/:

• order.rs — Order creation, validation, state transitions
• orders.rs — Order listing, filtering
• take_buy.rs — Taking buy orders
• take_sell.rs — Taking sell orders
• fiat_sent.rs — Fiat sent confirmation
• release.rs — Payment release logic
• cancel.rs — Order cancellation
• trade_pubkey.rs — Trade key management
• last_trade_index.rs — Trade index tracking

Goal

• Run cargo mutants --file src/app/order.rs --file src/app/take_buy.rs ... on each module
• Document baseline mutation score per module
• Kill surviving mutants by writing targeted tests
• Target: >70% mutation score for order/trade modules

Priority Mutants to Kill

• State transition logic (e.g., active → pending → success)
• Price/amount validation boundaries
• Fee calculation correctness
• Expiration checks
• Double-spend prevention guards

Acceptance Criteria

• Baseline mutation report for each module
• All critical mutants in state transitions killed
• All critical mutants in amount/fee validation killed
• Mutation score documented in PR
• No regression in existing tests

Ref: #618

[mostronatorcoder]

Baseline Mutation Testing Results

Date: 2026-03-10
Tool: cargo-mutants v27.0.0
Duration: ~75 minutes

Metric	Count
Total Mutants	104
✅ Killed	0
❌ Survived	98
⚪ Unviable	6
Score	0.0%

Breakdown by file

File	Survived
src/app/release.rs	46
src/app/cancel.rs	19
src/app/order.rs	18
src/app/take_sell.rs	7
src/app/take_buy.rs	6
src/app/fiat_sent.rs	2

Root Cause

All functions in these modules are async handlers that depend on global state:

• get_db_pool() — global DB pool
• Settings::get_mostro() / Settings::get_ln() — global config
• get_nostr_client() — global Nostr client
• enqueue_order_msg() — global message queue

These cannot be unit tested without dependency injection or mocking infrastructure.

Recommended Path

See refactoring issue (TBD) for making these modules testable via dependency injection. Until then, only pure helper functions within these modules can be targeted for mutation testing.

[mostronatorcoder]

Created #639 for the dependency injection refactoring needed to make these modules testable.