fix(token-swap): use pool_b amount (not pool_a squared) in invariant check

The post-swap invariant check in `handle_swap_exact_tokens_for_tokens`
was comparing against `pool_account_a.amount * pool_account_a.amount`
instead of `pool_account_a.amount * pool_account_b.amount`.

The constant-product invariant is `a * b = k`, not `a * a`. With the
bug, the check was effectively meaningless for any real swap because
the squared single-side amount bears no relation to the swap's
mathematical correctness — the check would spuriously pass or fail
depending on pool sizes rather than catching actual invariant
violations.

Split out from 25399b25 ("Speed up CI") — that commit bundled
unrelated CI workflow changes with this math fix; separating so each
can be reviewed in isolation.

Author: mikemaccana-edwardbot

tokens/token-swap/anchor/programs/token-swap/src/instructions/swap_exact_tokens_for_tokens.rs

@@ -134,7 +134,7 @@ pub fn handle_swap_exact_tokens_for_tokens(
     // We tolerate if the new invariant is higher because it means a rounding error for LPs
     context.accounts.pool_account_a.reload()?;
     context.accounts.pool_account_b.reload()?;
-    if invariant > context.accounts.pool_account_a.amount * context.accounts.pool_account_a.amount {
+    if invariant > context.accounts.pool_account_a.amount * context.accounts.pool_account_b.amount {
         return err!(TutorialError::InvariantViolated);
     }