Add tests for previously untested utility functions

[Copilot]

Several utility functions in pretext/utils.py lacked test coverage, and test_working_directory had an unresolved # TODO comment.

Changes

• test_working_directory: Completed the TODO — asserts the working directory is restored after the context manager exits
• test_format_docstring_as_help_str: Covers paragraph collapsing, double-newline preservation, whitespace stripping, and empty input
• test_is_earlier_version: Covers major/minor/patch comparisons, equality, and the post-release dev build convention (e.g. 2.11.5 < 2.11.5.dev0)
• test_hash_path: Asserts 10-char hex output, determinism, and uniqueness across distinct paths
• test_xml_syntax_is_valid: Covers well-formed PreTeXt, wrong root tag, malformed XML, and IOError on missing file

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Original prompt

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This section details on the original issue you should resolve

<issue_title>Consolidate and update testing issues</issue_title>
<issue_description>There are a few issues that relate to creating or improving tests. These should be consolidated or organized. Additions issues listing the other gaps in tests should be added.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Consolidate and update testing issues #1123

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