Negative numeric walltime durations are accepted

[tusharhqq]

What I observed

The walltime duration parser accepts a bare negative numeric duration such as -1 far enough that execution starts.

I expected a negative duration to be rejected as an invalid duration before benchmark execution begins.

Reproduction

I reproduced this against the exec harness walltime option from this checkout:

cargo run -p exec-harness -- --max-time -1 true

The command did not fail during duration parsing. It started running the benchmark and then failed later:

Executing: true
Completed 142 warmup rounds
Error: Unable to determine number of rounds to perform

I also checked the parser path directly with this local test:

#[test]
fn negative_numeric_duration_is_rejected() {
    assert!(parse_duration_to_ns("-1").is_err());
}

That assertion fails.

Expected behavior

Bare numeric durations should reject negative values.

Actual behavior

A bare numeric value like -1 is accepted by duration parsing and turns into a later execution error instead of an invalid-duration error.

I have not tried to fully classify every invalid numeric input here; the concrete case I observed is -1.