Add moderation API support to gateway provider

[tbille]

Summary

Adds content moderation support to the Go SDK's gateway provider, exposing the gateway's POST /v1/moderations endpoint through a new ModerationProvider interface. Introduces *errors.UnsupportedError so callers can cleanly detect backends (e.g. Anthropic) that do not offer moderation.

Changes

• Add ModerationParams, ModerationResult, ModerationResponse, ContentPart, ImageURL and the optional ModerationProvider interface in providers/types.go; add Moderation bool to providers.Capabilities.
• Add UnsupportedError, ErrUnsupported sentinel, CodeUnsupported constant, and NewUnsupportedError constructor in errors/errors.go to distinguish "provider does not support this operation" from the existing "provider not registered" case.
• Re-export the new types, interface, and sentinel from the root anyllm package to match the existing alias pattern.
• Implement Moderation on *gateway.Provider by calling POST /v1/moderations directly with the captured *http.Client, so IncludeRaw can be forwarded as ?include_raw=true and the gateway's locked "does not support moderation" / "multimodal moderation" 400 phrasing can be mapped to *UnsupportedError. Capture baseURL, httpClient and the platform token on Provider at construction so direct HTTP calls share the same auth as the embedded openai-go SDK. Advertise Moderation: true in capabilities() and add the providers.ModerationProvider interface assertion.
• Add httptest-based tests covering the happy path, the include_raw query parameter, both unsupported phrasings (moderation / multimodal) mapping to *UnsupportedError, status-code mapping for 401/402/429/502/504, platform and non-platform auth on the direct HTTP call, and an env-gated integration test.
• Add a README snippet showing the gateway provider's Moderation method and how to branch on UnsupportedError.

Testing

• go test ./errors ./providers/gateway -race -count=1 (targeted to packages changed)
• Integration test TestIntegrationModeration is env-gated on GATEWAY_API_BASE / GATEWAY_PLATFORM_TOKEN and skips when unset.
• Full suite runs in CI.

Checklist

• Linting passes (make lint)
• Tests pass locally (make test)
• Documentation updated (if needed)
• No breaking changes (or documented in summary)