fix: Update Mod Builder symbolic expression multiplication test (#1551)

The symbolic expression multiplication unit test was being run with an
incorrect expected result. The test became out-of-date after #1473
changed the `max_abs` method.

This PR updates the test with the correct expected result and adds the
Mod Builder crate's unit tests to the CI workflow.

This change has no impact on the functionality of the Mod Builder crate
as it only updates the tests.

Author: Avaneesh-axiom

.github/workflows/primitives.yml

@@ -9,6 +9,7 @@ on:
       - "crates/circuits/primitives/**"
       - "crates/circuits/poseidon2-air/**"
       - "crates/circuits/sha256-air/**"
+      - "crates/circuits/mod-builder/**"
       - "Cargo.toml"
       - ".github/workflows/primitives.yml"
 
@@ -50,3 +51,8 @@ jobs:
         working-directory: crates/circuits/sha256-air
         run: |
           cargo nextest run --cargo-profile fast --features parallel
+
+      - name: Run tests for mod-builder
+        working-directory: crates/circuits/mod-builder
+        run: |
+          cargo nextest run --cargo-profile fast --features parallel

crates/circuits/mod-builder/src/tests.rs

@@ -387,9 +387,9 @@ fn test_symbolic_limbs_mul() {
         Box::new(SymbolicExpr::Var(0)),
         Box::new(SymbolicExpr::Var(1)),
     );
-    // x * y = pq, q can be up to p so can limbs as p.
-    // x * y and p * q  both have 63 limbs.
-    let expected_q = 32;
-    let expected_carry = 63;
+    // x * y = pq, and x,y can be up to 2^256 - 1 so q can be up to ceil((2^256 - 1)^2 / p) which has 257 bits, which is 33 limbs
+    // x * y has 63 limbs, but p * q can have 64 limbs since q is 33 limbs
+    let expected_q = 33;
+    let expected_carry = 64;
     test_symbolic_limbs(expr, expected_q, expected_carry);
 }