Reject negative numbers in dehumanize()

[eeshsaxena]

Summary

Fixes #1278.

dehumanize() extracts numbers with an unsigned \d+ pattern, and the surrounding match anchors on a word boundary, so a minus sign attached to a number (e.g. "in -1 hours") is silently discarded. The result is a datetime in the opposite direction from what the string expressed.

Before

>>> now = arrow.now()
>>> (now.dehumanize("in 1 hours")  - now).total_seconds()
3600.0
>>> (now.dehumanize("in -1 hours") - now).total_seconds()
3600.0   # <- same as +1, the minus was dropped

After

>>> now.dehumanize("in -1 hours")
ValueError: Invalid input string: 'in -1 hours'. Negative numbers are not supported; use 'ago' or 'in' to indicate direction.

Rationale

Humanized strings encode direction with words ("ago"/"in"), never with an arithmetic sign, so humanize() itself never emits one. A negative number in the input is therefore always malformed, and returning a quietly-wrong result is worse than raising. The guard rejects any -<digit> in the input; all existing positive/ago/in/now inputs are unaffected.

Tests

Added test_negative_number (covers "in -1 hours", "in -2 days", "-3 minutes ago"). Full dehumanize suite passes (23 tests).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2224255) to head (baa58ee).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1310   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines         2315      2317    +2     
  Branches       358       359    +1     
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+ Hits          2315      2317    +2     

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