API Error Handbook (Top 30 Cases + Copy/Paste Fixes)

Our dev team repeats the same support answers around API errors. Create a pragmatic handbook covering the top ~30 issues with symptoms, probable causes, request/response snippets, and copy/paste remediation steps. Output in Markdown with consistent headers, anchors, and a small glossary. Include a quick-start decision tree for L1 support. Accuracy and clarity are critical.