### What / why
`TimeEstimateParse` (used by the issue time-estimate form) only checked
that the first token starts at the beginning of the string and the last
token ends at its end, but never checked the gaps between consecutive
tokens. Non-whitespace garbage embedded between two valid units was
silently dropped and the string accepted with a wrong value instead of
being reported as invalid.
Examples that were wrongly accepted before this change:
- `1h 2x 3m` → 3780s (parsed as 1h3m)
- `1h_2m` → 3720s
- `1h,1m` → 3660s
All three now return an "invalid time string" error, while valid inputs
such as `1h 1m 1s` and `1h1m1s` keep working.
### How
Reject any non-whitespace content between two matched units.
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Signed-off-by: TowyTowy <towy@airreps.link>
Signed-off-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
