test(expr): add coverage for Add, Mul, left-associativity, and null literal

[protosphinx]

Why

BinOp::Add, BinOp::Mul, and Literal::Null are implemented in the parser (src/parser.rs) but have zero tests that verify the correct AST output. The only existing use of RETURN 1 + 2 in the test suite (tests/prune.rs) checks output column names, not the operator itself. Literal::Null has no test at all. This PR fills that gap.

What

• 6 new tests appended to tests/expr.rs:
  • parses_addition: verifies RETURN 2 + 3 produces Binary { op: Add, lhs: Int(2), rhs: Int(3) }
  • parses_multiplication: verifies RETURN 4 * 5 produces Binary { op: Mul, lhs: Int(4), rhs: Int(5) }
  • mul_binds_tighter_than_add: RETURN 2 + 3 * 4 outer op is Add, rhs is Mul
  • add_is_left_associative: RETURN 1 + 2 + 3 lhs of outer Add is itself an Add
  • parses_null_literal: RETURN null produces Literal::Null
  • null_is_case_insensitive: NULL, Null, null all produce Literal::Null
• Updated the module doc comment to list the newly covered operators.

Tests

• cargo test - all tests pass (22 in tests/expr.rs, full suite clean)
• cargo clippy --all-targets -- -D warnings - clean
• cargo fmt --check - clean

Self-merge gate

• all CI checks pass
• LOC delta < 250 (diff is 110 lines)
• no public-API surface change (src/lib.rs not modified)
• no runtime-dependency additions
• no workflow file changes
• tests added or extended (tests/expr.rs, 6 new tests)

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