fix(bump): Preserve existing changelog header when changelog_merge_prerelease is used with cz bump --changelog

[edgarrmondragon]

Description

Fixes a regression introduced in v4.11.3 (370ac9d, #1700) where using cz bump --changelog with changelog_merge_prerelease = true results in the header of the existing changelog file being removed.

Checklist

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Generated-by: Claude Code 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

Documentation Changes

• Run uv run poe doc locally to ensure the documentation pages renders correctly
• Check and fix any broken links (internal or external)

Expected Behavior

Steps to Test This Pull Request

Additional Context

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.99%. Comparing base (2cfb8c6) to head (fb214be).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1850   +/-   ##
=======================================
  Coverage   97.99%   97.99%           
=======================================
  Files          60       60           
  Lines        2691     2693    +2     
=======================================
+ Hits         2637     2639    +2     
  Misses         54       54           

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[bearomorphism]

Thanks you for reporting this issue and providing a fix! A few nits

[bearomorphism]

These assertions aren't needed

[bearomorphism]

ditto

[bearomorphism]

Suggested change

	has_prereleases_to_merge = latest_full_release_info.index is not None and (
	latest_full_release_info.name is None
	or (
	changelog_meta.latest_version_position is not None
	and changelog_meta.latest_version_position
	< latest_full_release_info.index
	)
	)
	if has_prereleases_to_merge and latest_full_release_info.index is not None:
	if latest_full_release_info.index is not None and (
	latest_full_release_info.name is None
	or (
	changelog_meta.latest_version_position is not None
	and changelog_meta.latest_version_position
	< latest_full_release_info.index
	)
	):

While has_prereleases_to_merge is descriptive, the condition latest_full_release_info.index is not None is duplicated and removing it will make mypy unhappy. The above comment is enough to help reader understand the intention behind this long expression.