feat(json): Add JSON encoding options

[wklken]

Summary

• Add JSONOptions and SetJSONOptions for JSON request body encoding settings
• Support DisableHTMLEscape while preserving the default encoding/json behavior
• Document the new JSON option and cover large integer precision regression

Verification

• go test -run 'TestJSONOptionsControlsHTMLEscape|TestJSONSendPreservesLargeIntegerPrecision' -count=1 ./...
• go test -v ./...
• make test
• make lint
• git diff --check

[cursor]

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