fix(util): avoid panic when parsing a bare "s" duration

[BOURBONCASK]

Description

What does this PR do?

parse_duration (in zenoh-util's time DSL) panics on the input "s".

It reads the duration's trailing byte to detect the unit, and for s it reads a second byte to disambiguate s (seconds) from ms (milliseconds):

let mut it = s.bytes().enumerate().rev();
match it.next().unwrap() {                    // guarded by the leading is_empty() check
    ...
    (_, b's') => match it.next().unwrap() {   // <-- second next(): NOT guarded
        (i, b'm') => ... // "ms"
        (i, _)    => ... // "s"
    },
    ...
}

The leading is_empty() guard only covers the first next(). The input "s" (a unit with no numeric part) exhausts the iterator, so the second unwrap() panics with called Option::unwrap() on a None value. Every other unit-only input ("u", "m", "h", "d", "w") only calls next() once and already returns an Err; "s" is the only unit that double-consumes.

This change handles the exhausted-iterator case like the other unit-only inputs and returns a parse error instead of panicking. It also extends test_parse_duration with all unit-only inputs and end-to-end TimeExpr/TimeRange cases.

Minimal repro on main:

use zenoh_util::time_range::{TimeExpr, TimeRange};

let _ = "now(s)".parse::<TimeExpr>();      // panics
let _ = "[now(s)..]".parse::<TimeRange>(); // panics

Why is this change needed?

parse_duration is reachable from the public time DSL (TimeExpr / TimeRange), which backs the standardized _time selector parameter (ZenohParameters::time_range()). That value comes from the querier's selector, so a malformed _time (e.g. _time=[now(s)..]) is untrusted input that can panic the parsing side — both when sending a get() (ConsolidationMode::Auto reads time_range()) and on a queryable/storage that inspects _time.

The intent is clearly to fail gracefully: time_range() returns Option<ZResult<TimeRange>>, i.e. malformed input is supposed to yield an Err, not a panic. This follows the same "reject malformed input instead of panicking" direction as recent parsing fixes.

Related Issues

N/A — found via review of the time-DSL parsing path.

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[oteffahi]

Small nitpick. Otherwise LGTM.