Docs - BYOM fix predict example

[Raahim-Lone]

Description

This updates the BYOM docs example to avoid target leakage and clarifies that prediction inputs should not include the target column when joining for predictions. Primary testing should look at docs/integrations/ai-engines/byom.mdx.

Additional testing can start MindsDB locally, train a BYOM model on a small SQLite table using the updated example, and verify predictions work when the input query excludes the target column.

Fixes #11543

Documentation Update

(Please delete options that are not relevant)

• 📄 This change requires a documentation update

Verification Process

To ensure the changes are working as expected:

• Test Location: Specify the URL or path for testing.
• Verification Steps: Outline the steps or queries needed to validate the change. Include any data, configurations, or actions required to reproduce or see the new functionality.

Checklist:

• My code follows the style guidelines(PEP 8) of MindsDB.
• I have appropriately commented on my code, especially in complex areas.
• Necessary documentation updates are either made or tracked in issues.

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