Skip shell env noise when matching pathbase templates

[rsgalloway]

This pull request improves the robustness of template detection and error handling in the pathbase template logic. The main changes focus on ensuring that only valid path templates are considered, and that errors in template pattern compilation are handled gracefully. Additionally, a new test ensures that shell environment variables that do not represent file paths are ignored.

Template detection and validation:

• Updated _iter_template_items in lib/pathbase/template.py to use string.Formatter().parse() for validating template strings, ensuring only strings with valid template fields are considered, and added logic to skip invalid or non-template values. [1] [2]
• Added a test (test_find_matching_templates_ignores_shell_env_values) in tests/test_template.py to verify that shell environment variables (such as BASH_FUNC_module%%) are ignored when searching for matching templates.

Error handling improvements:

• Wrapped the call to _compile_pattern() in the Template class constructor with a try/except block to catch and re-raise regular expression errors as InvalidTemplateError, improving error reporting for invalid templates.