Duration try_from string parser accepts malformed signs and flips sign

Unconfirmed. This is the result of a search with Claude, may be a false positive.

In `src/wkt/src/duration.rs`, `Duration::try_from(&str)` parses duration strings:

https://github.com/googleapis/google-cloud-rust/blob/f02b42c2ac5720334cd2b5335824bd4166264527/src/wkt/src/duration.rs#L292-L316

The parser strips an initial leading `-` sign into `sign`, then parses the remaining string using `str::parse::` and `str::parse::`.

Because standard integer parsing accepts its own leading sign, malformed strings are accepted with incorrect values:
- `"--1s"`: `sign` is `-1`, remaining is `"-1"`; `(-1) * (-1)` parses as `+1s`.
- `"-+1s"`: parses as `-1s`.
- `"0.-5s"`: parses as `-0.05s`.
- `"5.s"`: parses as `5s`.

Start in src/wkt/src/duration.rs at the Duration::try_from(&str) implementation around lines 292-316, and reproduce the malformed examples listed in the issue. Trace how the leading sign and integer parsing interact; done means malformed signs and fractional forms are rejected while valid duration strings continue to parse correctly.

Written by the indexing model from the issue text.