Raise ValueError for negative numbers in dehumanize()

[awath-abdat]

Pull Request Checklist

• Added tests for changed code.
• All tests pass when run locally.
• All linting checks pass when run locally.
• Updated documentation for changed code.
• Code is up-to-date with the master branch.

Description of Changes

Closes: #1278

dehumanize() silently dropped the minus sign from inputs like "in -1 hours" because the number-extraction regex (\d+) only matches unsigned digits. The result was that "in -1 hours" produced the same output as "in 1 hours" — a quietly wrong datetime in the opposite direction from what a user would expect.

Since humanized time strings express direction via "ago"/"in" rather than arithmetic signs, negative numbers are never valid input. This PR adds an early validation check that raises ValueError when the input contains a negative number (e.g. -1, - 2).

Added test_negative_numbers covering three variants: "in -1 hours", "-3 minutes ago", and "in -2 days". All 23 dehumanize tests pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2224255) to head (c0cd576).
✅ All tests successful. No failed tests found.

Additional details and impacted files

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##            master     #1302   +/-   ##
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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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