Purpose

Closes #1402

Custom Jinja extensions configured via [tool.semantic_release.changelog.environment] were only available in changelog templates, but not in release notes templates.

This PR applies the same changelog.environment configuration when rendering custom release notes templates.

Rationale

The generate_release_notes() function was creating a separate, non-configurable Jinja environment that ignored user settings. This was inconsistent with how write_changelog_files() handles custom templates.

The fix follows the same pattern used for changelog templates:

• if custom template exists, use the user's configured template_environment
• otherwise use a plain default environment to ensure compatibility

How did you test?

test_release_notes_uses_custom_jinja_extension: Verifies custom Jinja extensions with filters are available in release notes templates. All existing release notes tests continue to pass.

How to Verify

1. Configure a custom Jinja extension in pyproject.toml:

   [tool.semantic_release.changelog.environment]
   extensions = ["my_package.ext:MyCustomExtension"]

2. Create a custom .release_notes.md.j2 template using a filter from the extension:

   {{ "text" | my_custom_filter }}

3. Run semantic-release changelog --post-to-release-tag <tag> or semantic-release version

4. Verify the release notes render correctly with the custom filter applied

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PR Completion Checklist

• Reviewed & followed the Contributor Guidelines

• Changes Implemented & Validation pipeline succeeds

• Commits follow the Conventional Commits standard
  and are separated into the proper commit type and scope (recommended order: test, build, feat/fix, docs)

• Appropriate Unit tests added/updated

• Appropriate End-to-End tests added/updated

• Appropriate Documentation added/updated and syntax validated for sphinx build (see Contributor Guidelines)