A5 - Remove PRECONDITION_ELEMENT token and collapse into varRef

[ruizmaa]

Closes #52

Summary

PRECONDITION_ELEMENT was an undocumented lexer token for v_xxx forms that duplicated the shape of varRef. The fix removes it and extends VAR_REFERENCE to cover both forms, routing everything through the existing varRef rule.

• Removed PRECONDITION_PREFIX fragment and PRECONDITION_ELEMENT token from dpm_xlLexer.g4
• Extended VAR_REFERENCE to match both vVAR_CODE and v_TABLE_CODE forms: VAR_REF_PREFIX (VAR_CODE | '_' TABLE_CODE)
• Removed the preconditionElem alternative from selectOperand in dpm_xlParser.g4 and deleted the preconditionElem rule
• Updated visitVarRef in constructor.py to detect the v_ prefix and return a PreconditionItem node (same node as before); removed the now-redundant visitPreconditionElem method
• Regenerated all files under grammar/generated/ using ANTLR 4.9.2 (antlr4 -Dlanguage=Python3 -visitor)

The AST output is unchanged: {v_Z101} still produces PreconditionItem(variable_id='Z101', variable_code='Z101'), so all downstream code (semantic analyser, ML generation, operands) requires no modifications.

Tests

Unit tests were added in tests/unit/ast/test_precondition_elem.py:

• test_valid_precondition_elem — parametrized over {v_Z101}, {v_SomeTable}, {v_A} (single-letter code), and {v_Some-Table} (hyphen in code, exercising the TABLE_CODE vs VAR_CODE difference); verifies all are accepted by the parser
• test_precondition_elem_produces_precondition_item — same inputs; verifies each produces a PreconditionItem with the correct variable_id and variable_code
• test_regular_var_ref — {vMyVar} continues to produce a VarRef node, confirming no regression in the normal variable reference path
• test_invalid_precondition_forms_are_rejected — parametrized over four invalid forms: bare {v_} (no code), {v_123} (digit-initial code), {v_Z101} as the head of a with-clause (not a valid cell ref), and {v_Z101} <- 1 (precondition element cannot be the LHS of a persistent assignment)

Checklist

• Code quality checks pass (ruff format, ruff check, mypy)
• Tests pass (pytest) with 100% branch coverage (coverage report --fail-under=100)
• Documentation updated (if applicable)

Impact / Risk

• Breaking changes? (public API / CLI / REST endpoints / Django models)
• Database schema or migration concerns?
• Notes for release/changelog?

Notes

[javihern98]

Looks good, thanks! 😊