🧹 Consolidate response decoding logic

[arvarik]

🎯 What: Extracted common request execution and API error checking logic into a new private method performRequestAndCheck in eero/client.go. Refactored do and doRaw to use this helper.
💡 Why: This reduces code duplication and improves maintainability by centralizing the error handling and envelope unmarshaling logic for the Eero API.
✅ Verification: Ran go test -v ./eero/... and confirmed all tests passed. The logic remains functionally identical, including special cases like null data handling.
✨ Result: A cleaner and more maintainable eero/client.go with centralized response processing.

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PR created automatically by Jules for task 4290333284834500035 started by @arvarik

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