Fix Result[T] status code inconsistency when both Code and Err are set

[Copilot]

When Result[T] has both Code and Err fields set, the HTTP status and error JSON body showed different codes:

// Before: HTTP 403, but body shows {"code":500,"error":"internal_error"}
return Result[User]{
    Code: 403,
    Err:  errors.New("access denied"),
}

Changes

• Reordered handleResult: Check for errors before writing status code to prevent double-write
• Added handleErrorWithCode: New function accepts explicit status code alongside error
• Status code validation: Validates codes are in 100-599 range, logs warning and falls back to inferred code if invalid
• Documentation: Added function comments explaining the delegation pattern

// After: Both HTTP status and JSON body show 403
func handleResult(w http.ResponseWriter, result Result[any]) error {
    if result.Headers != nil {
        WriteHeaders(w, result.Headers)
    }

    if result.Err != nil {
        if result.Code != 0 {
            return handleErrorWithCode(w, result.Err, result.Code)
        }
        return handleError(w, result.Err)
    }

    if result.Code != 0 {
        w.WriteHeader(result.Code)
    }

    return handleCommonTypes(w, result.Data)
}

Test Coverage

Added 7 test cases covering:

• Code + Err (explicit code used)
• Err only (code inferred)
• Code + Data (normal success)
• Invalid codes (validation + fallback)

Original prompt

分析该框架有没有bug，若有请修复，并提供对应的测试用例，提交前，请确保用例全部pass

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