[ElliotEtag]

Description:

• Preserve provider-reported usage alongside Google/Vertex JSON errors, including prompt blocks. This completes the JSON counterpart of fix: preserve Gemini refusals and zero-output usage #17's SSE usage handling.
• Reuse the existing JSON usage mapping, including reasoning, audio and Vertex cache-creation accounting. Errors without usage still omit it.
• Gen can then record provider cost without charging callers; Pollinations #18814 also handles failed fallback attempts.

Tests Run/Test cases added:

• 27 stream/response tests, 47 Vertex tests, 15 parameter tests; formatting, build and startup pass.
• Nine E2E cases through modified Gen and gateway: five live Google calls, plus four native fixtures. The documented seven-token JSON block returns 422, records provider cost and debits zero.
• Gen's 95 tracking tests and typecheck pass.
• Local wallet/telemetry; JSON uses the inline coordinator helper. Gateway typecheck retains 11 existing main diagnostics.

Type of Change:

• Bug fix

Rollout requires Pollinations #18814, which includes the gateway pin update. Provider invoicing for blocked prompts remains unverified; this preserves reported counters without changing caller billing.