#1465: require exactly three operands for award conditionals

[gemshrine]

Fixes #1465.

The bill evaluator rejected an if term only when it had fewer than three operands. Additional operands were discarded, and the bylaw formatter performed no arity check at all. A malformed rule could therefore produce a payment and description that omitted part of the supplied expression.

Both paths now require exactly three operands and report the number supplied when that requirement is not met. Correct conditionals keep their existing calculation and formatting behavior.

The regression covers zero, one, two and four operands through both billing and bylaw generation. The award, award-logic and between tests passed: 28 tests, 120 assertions. RuboCop passed for both changed files using rubocop-elegant 0.5.1 from the lockfile, and git diff --check passed.

[gemshrine]

This duplicates the existing fix in #1360, which validates the operand count of if in both calculation and bylaw rendering along with the other fixed-arity expressions. Keeping that existing PR avoids competing fixes for the same behavior.