Reject negative numbers in dehumanize instead of silently dropping the sign

[afonsojanu]

Fixes #1278

dehumanize() extracts the number for each matched time unit with an unsigned pattern (\d+), so a literal minus sign right before it, as in "in -1 hours" or "-3 minutes ago", is invisible to that regex. Direction already comes from matching the whole string against the locale's "ago"/"in" templates separately, so the sign just gets thrown away and the result lands in the opposite direction from what was written, with nothing raised:

now = arrow.now()
now.dehumanize("in 1 hours")   # +1 hour
now.dehumanize("in -1 hours")  # also +1 hour, same as above

Since direction is already conveyed by that phrasing, a signed number on top of it doesn't have a sensible fallback meaning, so this raises a ValueError instead. The check looks at the character in the original input immediately before the matched digits, since the number match itself never includes a preceding minus (word-boundary matching stops right at the digit).

Added test_negative_numbers_rejected covering the two cases from the issue plus "in -2 days", and confirmed unsigned input in both directions still behaves exactly as before. Verified the new test actually catches the bug by reverting just the source change and rerunning it, full suite (1903 tests) passes, and black/flake8/mypy/isort are all clean on the touched files.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2224255) to head (bff81bf).
✅ All tests successful. No failed tests found.

Additional details and impacted files

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##            master     #1344   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines         2315      2318    +3     
  Branches       358       359    +1     
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+ Hits          2315      2318    +3     

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[Copilot]

Pull request overview

This PR updates Arrow.dehumanize() to reject user input containing negative numeric values (e.g., "in -1 hours", "-3 minutes ago") which were previously parsed incorrectly due to unsigned digit matching, and adds regression tests to ensure the new behavior.

Changes:

• Add a validation check in Arrow.dehumanize() to detect and reject a literal - immediately preceding matched digits.
• Add a new test (test_negative_numbers_rejected) covering multiple negative-value inputs and confirming unsigned behavior is unchanged.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
arrow/arrow.py	Adds explicit rejection of negative numeric literals during dehumanize parsing.
tests/test_arrow.py	Adds regression test coverage for negative-number inputs and confirms existing unsigned behavior.

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