test: mutation testing for session restore module

[mostronatorcoder]

Context

Follow-up from #618. Implement mutation testing for session restore — ensures users can recover their trading sessions.

Scope

• src/app/restore_session.rs — Session restoration logic

Goal

• Run cargo mutants --file src/app/restore_session.rs
• Kill surviving mutants
• Target: >70% mutation score

Priority Mutants to Kill

• Session validation logic
• Order state recovery
• Permission checks (who can restore what)
• Data integrity verification

Acceptance Criteria

• Baseline mutation report
• Critical mutants killed
• Mutation score documented in PR

Ref: #618