SAS-172: Surface structured backend error messages instead of a generic fallback

[rjimeno-mz]

Summary

• Regenerates the OpenAPI schema for the new ApiError type (cloud#13055).
• Fixes a deeper bug found while wiring this up: openapi-fetch splits a response into data (2xx) or error (non-2xx), never both, but handleOpenApiResponse/handleOpenApiResponseWithBody only ever read data. A JSON error body could never reach the client regardless of backend changes. error is now an optional third parameter (backward compatible with all other call sites), wired through for getCostsBreakdownDaily.
• Adds getApiError() (src/api/openApiUtils.ts) to extract the structured body from a caught error.
• AccountSpendBreakdown now shows the backend's real message, and a request id (for non-validation reasons) instead of always showing a hardcoded generic string.

Test plan

• yarn typecheck / yarn lint
• New tests: structured error renders the real message + request id; validation errors render without a request id (nothing for support to look up); generic fallback still covers non-JSON/unmigrated error bodies
• Full billing/api suites pass (166 tests)

Resolves SAS-172 (merges SAS-149 and SAS-164). Follow-up for the remaining endpoints/call sites: SAS-174.

[linear-code]

SAS-172