feat(testing): add deterministic token/cost accounting to MockLLMBackend

[diiviikk5]

Summary

closess #1595

Add deterministic token/cost accounting to MockLLMBackend so test suites can assert usage drift and cost-related regressions without relying on external providers.

What Changed

• Added usage model types in tests/src/backend.rs:
  • InferenceUsage
  • UsageTotals
• Added deterministic accounting state to MockLLMBackend:
  • token pricing config
  • per-call usage history for successful inferences
• Added new APIs:
  • set_token_cost_rates(...)
  • reset_usage_accounting()
  • last_usage()
  • usage_history() / get_usage_history()
  • usage_totals() / get_usage_totals()
• Integrated accounting into successful infer() path.
• Added dedicated tests in tests/tests/backend_usage_tests.rs.

Why

This enables deterministic cost/usage assertions as part of the testing framework roadmap, which is important for regression testing, benchmark comparisons, and CI policy gating.

Testing

cargo test -p mofa-testing --test backend_usage_tests