feat: validate message_length_limit is non-negative

[SARAMALI15792]

Description

Added validation for message_length_limit to ensure it accepts only non-negative integers (>= 0). Negative values now raise InvalidCommandArgumentError instead of being silently treated as "no limit".

This PR addresses the inconsistency identified in #1909 where negative values were accepted but had unclear behavior.

Root Cause

After the fix in #1813 changed the default from None to 0 for "no limit", the validation logic still checked <= 0 instead of == 0. This meant:

• message_length_limit = 0 → no limit ✅ (intended)
• message_length_limit = -1 → no limit ✅ (unintended, should error)

Changes

• Validation: Added check in Check and Commit __init__ to reject message_length_limit < 0
• Logic fix: Changed condition from <= 0 to == 0 for "no limit" check
• Type updates: Changed CommitArgs and CheckArgs to accept int | None for proper CLI override handling
• Tests: Added comprehensive test coverage for negative value rejection in both commands
• Documentation: Updated docs/config/check.md and docs/config/commit.md to clarify non-negative requirement

Why This Is Safe

• Default is 0 (no limit), so existing configs without this setting are unaffected
• Negative values currently work as "no limit" but this is undocumented behavior
• Users relying on negative values can simply change to 0 (same effect, clearer intent)
• Aligns with the v4.13.0 design where 0 means "no limit"

Expected Behavior

• message_length_limit < 0 → raises InvalidCommandArgumentError ❌
• message_length_limit = 0 → disables length limit ✅
• message_length_limit > 0 → enforces the specified limit ✅
• CLI None value → falls back to config setting ✅

Steps to Test This Pull Request

# Test negative value raises error
echo "feat: hello" | cz check -l -1
# Expected: "message_length_limit must be a non-negative integer"

# Test zero disables limit
echo "feat: this is a very long message" | cz check -l 0
# Expected: "Commit validation: successful!"

# Test positive limit works
echo "feat: hello" | cz check -l 50
# Expected: "Commit validation: successful!"

# Run test suite
uv run poe test

Checklist

• Add test cases to all the changes introduced
• Run uv run poe all locally to ensure this change passes linter check and tests
• Manually test the changes:
  • Verify negative values raise error
  • Verify zero disables limit
  • Verify positive limits work
  • Verify CLI overrides config
  • Verify config fallback works
• Update the documentation for the changes

Related

• Fixes Discussion: Clarify / standardize behavior for negative message_length_limit (should it require >= 0?) #1909
• Related to Feat: Validate message_length_limit is non-negative #1908 (similar approach, independent implementation)
• Builds on fix(message_length_limit): align the behavior of message_length_limit #1813 (the fix that standardized 0 as "no limit")

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.99%. Comparing base (7c21c21) to head (fdcdaec).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1916   +/-   ##
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  Coverage   97.99%   97.99%           
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  Files          60       60           
  Lines        2689     2695    +6     
=======================================
+ Hits         2635     2641    +6     
  Misses         54       54           

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