fix(core): ignore malformed model costs

[nexxeln]

Summary

• treat malformed or non-finite model cost fields as zero during V2 usage accounting
• preserve valid pricing components and existing finite-price behavior
• add a regression for provider-supplied object values

V2 counterpart to #43248. Related to #43098.

Testing

• bun test test/session-runner.test.ts in packages/core (152 tests)
• bun typecheck in packages/core
• full workspace typecheck from the pre-push hook