fix(exec-harness): reject negative and non-finite numeric durations

[mohamedAtoui]

Fixes #364.

Bare numeric walltime durations were parsed with f64::from_str and converted with a saturating as u64 cast. That silently turned -1, -0.5 and NaN into 0, and inf into u64::MAX. Since a max_time of 0 means "unlimited", --max-time -1 ran the warmup phase and only failed afterwards with Unable to determine number of rounds to perform.

The numeric branch now builds a Duration with Duration::try_from_secs_f64, which already rejects negative, NaN and infinite values, and both branches share the single nanosecond conversion at the end. The error surfaces during argument validation, with the standard library explaining the cause:

Error: Invalid max_time

Caused by:
    0: Invalid duration: '-1'. Expected a finite, non-negative number of seconds
    1: cannot convert float seconds to Duration: value is negative

Humantime inputs such as -1s were already rejected by humantime::parse_duration, so that branch is unchanged.

Note for reviewers: -0 parses as -0.0, which try_from_secs_f64 accepts, so it stays valid and yields 0. That is intentional, 0 is the documented way to disable warmup, and it is covered by a test.

Adds unit tests for the rejected values, for 0/-0 remaining valid, and one covering the reported --max-time -1 path end to end.

[greptile-apps]

Greptile Summary

This PR tightens wall-time duration validation before converting numeric seconds to nanoseconds.

• Rejects negative, NaN, and infinite bare numeric durations.
• Preserves documented 0 and intentional -0 behavior.
• Adds direct parser tests and coverage of negative max_time through the production argument-conversion path.

Confidence Score: 5/5

The PR appears safe to merge; the new validation rejects the targeted invalid inputs while preserving documented zero-duration semantics.

No actionable failures remain: the guard runs before the saturating cast, the production argument-conversion path propagates the validation error, and tests cover rejected values plus 0 and -0.

Important Files Changed

Filename	Overview
crates/exec-harness/src/walltime/config.rs	Adds correct finite/non-negative validation and focused regression tests without changing valid duration behavior.

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