Brief a coding agent to add tests before a refactor

Goal: Write tests for {{module_path}} that pin down its current behavior, so that it can be refactored later without silently changing what it does.

Context:
- Test framework and command: {{test_command}}
- Existing test file to use as the pattern for style, fixtures and naming: {{pattern_test_file}}
- What this code is for and who calls it: {{module_purpose}}
- Cases I know matter: {{known_cases}}

Constraints:
- Do not change the code under test. If you find a bug, write a test that documents the current behavior, mark it clearly (for example with the framework's skip or expected-failure marker and a one-line comment), and report it. Do not fix it in this task.
- Test behavior through the public interface, not private helpers.
- No network calls, real clocks, or random values without a fixed seed. Use the fakes and fixtures the project already has, and do not add new test dependencies.
- Put new tests where the project keeps them, named like the pattern file (for example module_name.test.ts or tests/test_module_name.py).

Cover, in this order: the main success paths; boundary values (empty, zero, the maximum, one past the maximum); invalid input and error handling; then any branch in the code that no test reaches yet. Write at least 8 focused tests. Each test checks one behavior, and its name says what that behavior is, such as rejects_quantity_above_stock_limit.

Done when:
- All new tests pass with {{test_command}}, and the existing tests still pass.
- Each new test would fail if its behavior broke. Check this for at least two of them by temporarily breaking the code, then restore it.

Verify: run the full test command and include the last 20 lines of its output in your final message, followed by a table with the columns Test and Behavior checked. If the module cannot be tested without changing it, stop and describe the smallest change that would help before making it.