Function parse_duration_string 

fn parse_duration_string(s: &str) -> Option<Duration>

Parses a relative time string like "1h" or "30m" into a Duration.

Accepts a positive integer followed by exactly one unit character:

Suffix	Unit
s	seconds
m	minutes
h	hours
d	days

Returns None for any malformed input (missing digits, unknown unit, overflow). Callers fall back to endpoint-specific defaults.