fix: converters reject signed resource values to match validator behavior

[immanuwell]

Pull Request Checklist

• Fixes #
• Tests added
• Documentation/examples added
• Good commit messages and/or PR title

Description of PR

Currently, _convert_decimal_units and _convert_binary_units accept signed values like -500m or +1Gi due to [+-]? in their regex patterns. The validators (_validate_decimal_units, _validate_binary_units) correctly reject these, so there's a mismatch - same input, different outcome depending on which function you hit first.

This PR removes [+-]? from both converter patterns so they're consistent with validators. Also fixes _merge_dicts raising bare Exception instead of ValueError.

Reproduce:

from hera.workflows.converters import _convert_decimal_units, _convert_binary_units
from hera.workflows.validators import _validate_decimal_units

_validate_decimal_units("-500m")   # raises ValueError (correct)
_convert_decimal_units("-500m")    # returns -0.5 (bug)

_convert_binary_units("-512Mi")    # returns -536870912.0 (bug)

Negative resource values aren't valid in Kubernetes anyway, so rejecting them in both layers makes sense.

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.7%. Comparing base (8f8a626) to head (3ffab33).

Files with missing lines	Patch %	Lines
src/hera/workflows/resources.py	0.0%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #1588   +/-   ##
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  Coverage   80.7%   80.7%           
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  Files         62      62           
  Lines       5081    5081           
  Branches     771     771           
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  Hits        4101    4101           
  Misses       846     846           
  Partials     134     134           

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