Support multi-line commit body input

[axkoro]

Description

The interactive commit prompt had no way to enter multi-line commit bodies without resorting to --edit or using the newline separator | which is completely undocumented, from what I can tell.

This PR enables multi-line input for the commit body by setting multiline=True on the body question. This uses Questionary's native multi-line input. I also added a custom key binding, so that Enter submits on an empty buffer (skipping the field). Otherwise it now inserts a newline. Non-empty input can be submitted with Alt+Enter (Questionary's default).

Checklist

• I have read the contributing guidelines

Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)

Generated-by: Claude following the guidelines

Code Changes

• Add test cases to all the changes you introduce
• Run uv run poe all locally to ensure this change passes linter check and tests
• Manually test the changes:
  • Verify the feature/bug fix works as expected in real-world scenarios
  • Test edge cases and error conditions
  • Ensure backward compatibility is maintained
  • Document any manual testing steps performed
• Update the documentation for the changes
  • No updating required, from what I can tell.

Steps to Test This Pull Request

Added: entering multi-line commit body

1. Run cz commit and proceed to the body prompt
2. Type something, press Enter → a newline should be inserted
3. Type another line, press Alt+Enter → the input should be submitted with both lines preserved in the commit body

Kept: skipping the commit body

1. Run cz commit and proceed to the body prompt
2. (Enter nothing)
3. Press Enter → the field should be skipped

Additional Context

The body prompt now displays a > prefix, which is prompt_toolkit's default multi-line indicator. This is slightly inconsistent with the other input fields. Remove it or keep this and maybe even add it to the other inputs as well?

Resolves #1917

[codecov]

Codecov Report

❌ Patch coverage is 82.35294% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.89%. Comparing base (7c21c21) to head (aa3c061).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
commitizen/question.py	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1919      +/-   ##
==========================================
- Coverage   97.99%   97.89%   -0.10%     
==========================================
  Files          60       60              
  Lines        2689     2705      +16     
==========================================
+ Hits         2635     2648      +13     
- Misses         54       57       +3     

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