chore(librarian): Create package changelog if not present

[parthea]

When adding a new API version to an existing library, the contents of the existing package level CHANGELOG.md file is removed. The issue is that a new changelog is created unconditionally here:

google-cloud-python/.generator/cli.py

Line 295 in c7696a6

_create_new_changelog_for_library(library_id, output)

As an example, the command below adds the client library code for google/cloud/maintenance/api/v1 to an existing package google-cloud-maintenance-api

librarian generate -api=google/cloud/maintenance/api/v1 -library=google-cloud-maintenance-api

After running the command locally, see that the contents of this CHANGELOG.md is gone. See below

(py392) partheniou@partheniou-vm-3:~/git/google-cloud-python$ git diff packages/google-cloud-maintenance-api/CHANGELOG.md
diff --git a/packages/google-cloud-maintenance-api/CHANGELOG.md b/packages/google-cloud-maintenance-api/CHANGELOG.md
index 6436543a6e6..d884ea3cedb 100644
--- a/packages/google-cloud-maintenance-api/CHANGELOG.md
+++ b/packages/google-cloud-maintenance-api/CHANGELOG.md
@@ -3,31 +3,3 @@
 [PyPI History][1]
 
 [1]: https://pypi.org/project/google-cloud-maintenance-api/#history
-
-## [0.2.0](https://github.com/googleapis/google-cloud-python/compare/google-cloud-maintenance-api-v0.1.1...google-cloud-maintenance-api-v0.2.0) (2025-10-20)
-
-
-### Features
-
-* Add support for Python 3.14  ([98ee71abc0f97c88239b50bf0e0827df19630def](https://github.com/googleapis/google-cloud-python/commit/98ee71abc0f97c88239b50bf0e0827df19630def))
-
-
-### Bug Fixes
-
-* Deprecate credentials_file argument  ([98ee71abc0f97c88239b50bf0e0827df19630def](https://github.com/googleapis/google-cloud-python/commit/98ee71abc0f97c88239b50bf0e0827df19630def))
-
-## [0.1.1](https://github.com/googleapis/google-cloud-python/compare/google-cloud-maintenance-api-v0.1.0...google-cloud-maintenance-api-v0.1.1) (2025-07-23)
-
-
-### Documentation
-
-* Add missing comments for messages ([f955689](https://github.com/googleapis/google-cloud-python/commit/f9556891d9224fefd09202539a7d5830f724e2c4))
-
-## 0.1.0 (2025-06-23)

With the fix in this PR, a package level changelog will only be created if it is not present. An existing changelog will remain untouched.

[gemini-code-assist]

Summary of Changes

Hello @parthea, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces a crucial fix to the librarian tool, preventing the accidental deletion of existing package-level CHANGELOG.md files. Previously, when generating new API versions for existing libraries, the tool would unconditionally overwrite the changelog, leading to loss of release history. The updated logic ensures that a new changelog is only created if one does not already exist, thereby preserving valuable historical information.

Highlights

• Bug Fix: Resolved an issue where adding a new API version to an existing library would unintentionally delete the contents of its CHANGELOG.md file.
• Conditional Changelog Creation: The _create_new_changelog_for_library function in cli.py now only executes if the target library's directory (and thus its changelog) does not already exist, preventing data loss.
• Path Resolution: Introduced logic to correctly determine the library's path, accounting for monorepo structures, to ensure the existence check is accurate.

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[gemini-code-assist]

Code Review

This pull request addresses an issue where CHANGELOG.md was being overwritten when adding a new API version to an existing library. The fix correctly checks if the library directory exists before creating a new changelog, which effectively prevents this issue. The logic is sound. I've included one suggestion to use pathlib more idiomatically for path construction, which will improve code clarity and robustness.

[gemini-code-assist]

For improved readability and cross-platform compatibility, it's better to use pathlib's / operator for joining path components rather than f-string formatting. This is the idiomatic way to work with pathlib and avoids potential issues with path separators.

Suggested change

	path_to_library = f"packages/{library_id}" if is_mono_repo else "."
	if not Path(f"{repo}/{path_to_library}").exists():
	path_to_library_str = f"packages/{library_id}" if is_mono_repo else "."
	if not (Path(repo) / path_to_library_str).exists():