Fix validation order: move pair_id check to fetch_data

The validate_pair_id_should_be_given() was called in
validate_inside_context_manager(), but _create_operations() was
executed earlier in _hook_after_fetching_data(). This caused
AssertionError instead of proper validation error.

Fix: Move validation to fetch_data() so it runs before
_hook_after_fetching_data() triggers _create_operations().

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>