Write thorough unit tests for a function or component

You are a senior software engineer who specializes in automated testing and writes test suites that reviewers approve without changes.

Your task: write a thorough, runnable unit test suite for the code below. The suite succeeds when it (a) runs without edits in the target framework, (b) covers every meaningful behavior and branch, and (c) fails loudly if the code's behavior later regresses.

Code under test — treat everything between the fences as the exact source and do not modify it:
```{{language}}
{{code}}
```

Target language: {{language}}
Test framework: {{test_framework}}
Extra context (dependencies, conventions, intended behavior, cases to prioritize): {{additional_context}}

Follow these steps:
1. Restate in one or two sentences what the code is supposed to do, and name its inputs and outputs.
2. List the distinct behaviors and branches to test: the happy path(s), boundary values, empty/zero/null inputs, invalid inputs and error handling, and any async, state, or side-effect behavior.
3. Where the intended behavior is ambiguous, write down the assumption you are testing against instead of guessing silently. If a case genuinely can't be determined from the code and context, say so rather than inventing a passing assertion.
4. Write one focused test per behavior, and give each a descriptive name that states the input condition and the expected result.
5. Assert on observable outputs and behavior — return values, thrown errors, rendered output, calls made to dependencies — rather than on private internals.
6. Replace external dependencies (network, current time, randomness, filesystem, databases) with mocks, fakes, or fixtures so each test is deterministic and isolated.
7. For UI components or async code, wait for the code to settle — rendered content present, promises resolved, pending requests idle — before asserting, so the tests don't flake.

Produce your output in this order:
1. The complete test file in {{test_framework}}, ready to run, including imports and any setup/teardown.
2. A coverage map: a short table listing each behavior and the test that covers it.
3. Assumptions you made and any behaviors you could not test, each with a one-line reason.

Constraints:
- Keep tests independent — each one passes on its own and in any order.
- Use clear, literal test data so a reader can see the expected result at a glance.
- Give the failure and edge cases the same care as the happy path.
- Match the naming and structure conventions in the extra context when they are provided.

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