Document deprecation of ModelETL#getModel()

[anukalp2804]

Clarified that getModel() is a temporary legacy accessor and will be removed once transform() is fully implemented. Added guidance to rely on the extract -> transform -> load pipeline instead.

This work is part of Issue #1436 – Document Deprecations and improves the deprecation documentation for ModelETL#getModel().

• Confirmed this method is intentionally deprecated as a transitional API
• Added Javadoc explaining why it is deprecated and what to use instead
• No functional behavior changes
• Successfully verified build using mvn clean install

Reference: #1436

/cc @elharo

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