fix: BI-0 raise an error on unknown fields in BEFORE FILTER BY or IGNORE DIMENSIONS

[MCPN]

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Closes #531

Long time no see 😁

[Copilot]

Pull request overview

This PR adds validation to raise an error when unknown fields are referenced in BEFORE FILTER BY or IGNORE DIMENSIONS clauses, fixing issue #531. The solution introduces a new BFBChecker validator that verifies all field references in these clauses exist in the current field set.

Key Changes

• Added new BFBChecker validator to detect unknown field references in BEFORE FILTER BY clauses
• Introduced UnknownBFBFieldError exception type for reporting validation failures
• Enhanced LOOKUP function mutation to propagate errors from IGNORE DIMENSIONS clauses

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
lib/dl_formula/dl_formula/validation/bfb.py	Implements new BFBChecker validator class that validates field references in BEFORE FILTER BY clauses
lib/dl_formula/dl_formula/core/exc.py	Adds UnknownBFBFieldError exception for reporting unknown field references
lib/dl_query_processing/dl_query_processing/compilation/formula_compiler.py	Integrates BFBChecker into formula validation pipeline
lib/dl_formula/dl_formula_tests/unit/validation/test_bfb.py	Comprehensive test suite covering valid and invalid BEFORE FILTER BY scenarios
lib/dl_formula/dl_formula/mutation/lookup.py	Adds error propagation for IGNORE DIMENSIONS with unknown fields
lib/dl_api_lib/dl_api_lib_tests/db/data_api/result/complex_queries/test_window_functions.py	Removes xfail marker now that the issue is fixed
lib/dl_api_lib/dl_api_lib_tests/db/data_api/result/complex_queries/test_lookup_functions.py	Removes xfail marker and unused pytest import
lib/dl_formula/dl_formula/validation/window.py	Removes unused import
lib/dl_formula/dl_formula/validation/aggregation.py	Removes unused import

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