Function generate_msg_value 

pub fn generate_msg_value(test_runner: &mut TestRunner) -> Uint<256, 4>

Generates a msg.value for payable functions using TestRunner’s RNG (corpus mutation path).

Mirrors fuzz_msg_value by sampling from UintStrategy. The 15% mutation gate is applied at the call site in corpus.rs. Over-budget values are clamped to sender balance at execute time.