Add OpenAPI validation errors section

[mintlify]

Added comprehensive troubleshooting documentation for OpenAPI validation errors including common error types, fixes, and a step-by-step workflow. This helps users understand and resolve validation errors when running mint openapi-check or mint dev.

Files changed

• api-playground/troubleshooting.mdx - Added new "OpenAPI validation errors" section with detailed examples and troubleshooting steps

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Note

Adds comprehensive guidance to api-playground/troubleshooting.mdx for diagnosing and fixing OpenAPI validation issues.

• Explains error path format in validation messages and how to locate issues
• Details response schema errors (missing $ref/content/description) with incorrect vs. correct JSON examples
• Covers common $ref problems (nonexistent components, unsupported external refs) and corrections
• Provides a concise troubleshooting workflow: identify location, check spec, validate in Swagger Editor, apply fixes, re-run checks
• Adds links to relevant OpenAPI spec sections and tools

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