PRO-3522: Publish an actionable error reference

[dev-seth-vapi]

Description

• Adds an Errors and troubleshooting section under Support, with a recovery-first overview and request-ID guidance.
• Documents the additive API validation response, including the legacy fields, eight stable field issue codes, and JSON Pointer locations.
• Publishes all 91 existing dashboard diagnostic codes with their meaning, next step, and retry behavior.
• Links the new reference from Support, issue reporting, debugging, custom-tool troubleshooting, and call ended reasons.

Linear: PRO-3522

Testing Steps

• npx --yes fern-api@5.59.0 docs md check — all 376 MDX files are valid
• npx --yes fern-api@5.59.0 check --warnings — 0 errors; the same 5 existing warnings as main
• Compared the reference against the runtime catalog — 91 documented, 0 missing, 0 extra, 0 duplicates
• Parsed fern/docs.yml and ran git diff --check
• Rendered /errors, /errors/diagnostic-codes, and /errors/validation-errors with local Fern; all three returned 200, and the validation page was checked in light and dark themes

[github-actions]

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