drop mapToStruct and spaceResultToJSON, both dead with no duplicate call sites

[gerardrecinto]

Two more from the U1000 pile.

mapToStruct (api_core.go): a JSON-roundtrip map-to-struct helper marked 'for adapters,' zero callers. Checked whether any other code does the same marshal/unmarshal pattern inline that should be wired to it instead - found one similar-looking spot (expandNativePromptArgs in engine_native.go) but it's a map-to-map normalization with different fallback semantics, not the same thing, no real duplication to fix.

spaceResultToJSON (api_space.go): a struct-to-JSON-string helper marked 'for backward compatibility,' zero callers, and the only json.Marshal call in the whole file is its own.

Both removed along with the now-unused encoding/json imports in each file. Build+vet+gofmt+test clean.

[github-actions]

Gemini PR Review

The diff removes two helper functions (mapToStruct and spaceResultToJSON) and their corresponding encoding/json imports from api_core.go and api_space.go respectively.

Assuming these functions were either unused or their functionality has been correctly refactored and replaced elsewhere in the codebase, this diff looks good.

• Code quality and readability
  • Removes seemingly unused or redundant helper functions, which improves code cleanliness and reduces boilerplate.