Fix clang-format violations in proof rule string mapping

[Copilot]

The Check code formatting GitHub Actions job failed due to clang-format violations in src/api/cpp/cvc5_proof_rule_template.cpp. The failure was isolated to two ProofRule switch cases introduced with multi-line formatting that does not match repository clang-format expectations.

• Formatting-only correction in proof rule template
  • Normalized two case statements to clang-format-compliant single-line form in toString(ProofRule rule):
    • ProofRule::ARITH_POW2_MONOTONE
    • ProofRule::ARITH_POW2_LOWER_BOUND
  • No semantic or control-flow changes.

case ProofRule::ARITH_POW2_MONOTONE: return "ARITH_POW2_MONOTONE";
case ProofRule::ARITH_POW2_LOWER_BOUND: return "ARITH_POW2_LOWER_BOUND";