[codex] Fix OpenAI costs parsing

[SergeyLavrentev]

Summary

• Decode OpenAI organization cost amount.value when it is returned as a numeric string.
• Keep existing numeric decoding support for documented numeric responses.
• Add a regression fixture covering the string value shape observed from the live API.

Fixes #999

Root cause

The OpenAI Costs API can return amount.value as a string such as "12.50". OpenAIAPIUsageFetcher decoded this field as Double?, so a successful HTTP 200 costs response failed JSON decoding and OpenAI Admin API usage rendered an error.

Validation

• Live probe against /v1/organization/costs with an Admin API key decoded 31 cost buckets using the tolerant decoder.
• swift test --filter OpenAIAPICreditBalanceTests could not complete in this local environment because only Command Line Tools are installed; the macOS test target pulls KeyboardShortcuts, whose SwiftUI #Preview usage requires PreviewsMacros from full Xcode.

[steipete]

Thanks for the clean repro and patch. I landed the fix on main in f23a478 with the same tolerant decoding behavior, plus a small test tweak so the fixture covers both documented numeric values and the observed numeric-string value.

Validation:

• swift test --filter OpenAIAPICreditBalanceTests
• make check
• Codex review: no actionable findings

Closing this draft PR since the fix is now shipped on main.