effect: decide 402 → QuotaExhaustedError and carrying raw usage #323

Two error/usage gaps in @coder/ai-sdk-effect were left out of the publishing PR because each one changes behavior and needs a design choice. RateLimitError.retryAfter did ship there.

1. 402 → QuotaExhaustedError. Today HTTP 402 maps to RateLimitError, classifyError returns rate-limit, and isTransient returns true. Effect 4's QuotaExhaustedError exists for exhausted quota, but its isRetryable is false (effect/ai/AiError). Mapping 402 to it is inconsistent with isTransient: true, unless we also change the classification (a new quota reason, or rate-limit but not transient). That changes the public ErrorReason / isTransient contract, so it needs a decision.
2. Carry usage.raw. LanguageModelV4Usage has provider-raw usage, but Effect 4.0.3's Response.Usage has no raw field. Carrying it needs a new home, for example the finish part's metadata.coder, which is a public metadata shape.

Acceptance: a decision for each item, then a PR with unit tests and README updates (the error-reason table in packages/effect/README.md).

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