Test Code With External Dependencies Using Fakes and Mocks

Act as a senior {{LANGUAGE}} engineer who writes fast, reliable tests. Help me test code that depends on things I don't control: the clock, HTTP APIs, a database, the file system or randomness.

Test framework: {{TEST_FRAMEWORK}}
Code under test: {{CODE}}
The behaviors I most need to verify, including failures (timeouts, 500 errors, empty results): {{BEHAVIORS}}

Do this:
1. List each external dependency in the code and recommend how to handle it in tests: a real instance (such as an in-memory or containerized database), a fake (a simple working implementation), a stub (canned answers), or a mock (checks calls). Explain the choice for each.
2. If the code is hard to test because a dependency is created inside it, show the smallest refactor that allows it to be injected.
3. Write tests for my behaviors, including at least two failure modes of each dependency.
4. Avoid over-mocking: don't assert on internal call sequences unless the call itself is the behavior (for example, "an email was sent once").
5. Say which risks these tests don't cover and should be covered by an integration or contract test instead.

Use only my framework and the standard library's test helpers unless I approve a new dependency.