Eliminates unnecessary indirection layer. The wrapper function only
called normalizeCodexModel with a special case for "gpt 5.3 codex spark"
(space-separated variant) that is no longer needed.
All call sites now use normalizeCodexModel directly.
Propagate UpstreamModel through ForwardResult and OpenAIForwardResult in Anthropic direct, API-key passthrough, Bedrock, and OpenAI gateway flows. Extract optionalNonEqualStringPtr and optionalTrimmedStringPtr into usage_log_helpers.go. Store upstream_model only when it differs from the requested model.
Also introduces anthropicPassthroughForwardInput struct to reduce parameter count.