[Spectrum]: expand error codes with causes and resolutions

[aberglund-cf]

Summary

Expand the error docs for code which need explanation beyond their error message. Each section includes the cause and actionable resolution verified against the Spectrum API service.

Documentation checklist

• The change adheres to the documentation style guide

[cloudflare-docs-bot]

Review

⚠️ 2 warnings found in commit 5eeeeeb.

Code Review

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✅ No code review issues found.

Style Guide Review

Warnings (2)

File	Issue
spectrum/reference/error-codes.mdx line 155	Heading begins with a verb ending in -ing — Added line is ## Addressing errors (12xxx) Fix: Use the imperative form: ## Address errors (12xxx)
spectrum/reference/error-codes.mdx line 177	Bulleted list used for sequential steps — The \*\*Resolution:\*\* list under 16001 — Zone mismatch uses bullet points for ordered actions (Verify → Use → If → Contact). Fix: Convert the four-step resolution to a numbered list.

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