Fix: Only selected models need to be promoted when --inlclude-unmodified flag is provided

[izeigerman]

Fix #5516

[Copilot]

Pull Request Overview

This PR fixes an issue where all models were being promoted when the --include-unmodified flag was used, even when specific models were selected. The fix ensures that only explicitly selected models are promoted when both flags are combined.

Key changes:

• Updated promotion logic to respect model selection when --include-unmodified is provided
• Restructured conditional logic to properly handle the combination of include_unmodified and selected_models_to_backfill
• Added comprehensive test coverage for both --include-unmodified alone and combined with model selection

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/core/integration/test_plan_options.py	Added two new test cases to verify promotion behavior with include_unmodified flag both standalone and combined with model selection
sqlmesh/core/plan/definition.py	Refactored promotion logic to correctly determine which snapshots should be promoted based on the combination of flags provided

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