docs: add header validation section to headers documentation

[inkeep]

Summary

Documents the new custom headers validation feature added in PR #1699.

Changes

• Adds a new "Header Validation" section to /visual-builder/headers.mdx explaining:
  • Headers are validated in real-time against the configured schema
  • Invalid headers disable chat input and display an error message
  • Users must fix validation errors before sending messages

Related PR

• feat(dashboard): Notify users to enter custom headers if required in chat or if custom headers are invalid #1699 - feat(dashboard): Notify users to enter custom headers if required in chat or if custom headers are invalid

cc @dimaMachina

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