Fix #85: Support provider cost metering and accumulate in ledger

[AnandkumarMall]

Fixes #85. Extends the usage-reporting contract so an API provider adapter can return a cost field alongside tokens. This includes updating ModelResult and AgentRunResult to carry a cost payload, extracting cost from CodexEventCollector and ModelProviderAdapter, and updating the RuntimeUsageCollector and execution_guards to allow informational cost units (like cost_usd or neurons) to be recorded in the BudgetLedger without raising unapproved grant exceptions. The metrics are fully additive and correctly surface in system_run_summary.

[electrixoul]

Thank you, @AnandkumarMall, for contributing provider cost metering and ledger accumulation.

We transferred the cost-reporting work into validation/budget-and-cost-accounting and reconciled it with the active budget-admission contribution. The combined implementation preserves positional runtime/plugin compatibility, records provider-reported cost as an informational ledger unit, and does not silently convert every token-detail field into an unapproved budget unit. The branch passed the complete offline test suite (3,084 tests, 6 skipped), local CI/guardrails, coverage gates, documentation checks, and compile/diff hygiene before it was pushed.

Our maintainer workflow is documented here: https://github.com/sapientinc/PRAXIST/blob/main/docs/guides/contributing.md#what-happens-after-submission

This branch is now ready for a separately scheduled real-task experiment stage. Please allow us time to complete that validation; if it passes, the maintainers will submit the validated change to main with your authorship retained. We are closing this source PR only because its contribution has been transferred to the shared validation branch, not because the contribution was rejected.

[Glen-SP]

Update: this shipped. Your provider cost metering and ledger accumulation are now
on main in 9d6932d, merged via #199, with your authorship retained as a
co-author trailer.

Thank you for your contribution, @AnandkumarMall