Fix step creation mock to return 201+Location

[davegomez]

The base POST steps mock handler returned 200 with a JSON body, but
the real Fizzy API returns 201 with a Location header and empty body.
This meant the client's 201+Location follow logic in request() was
never exercised by the default mock — only by per-test overrides.

Now the base handler stores the created step via lastCreatedStep and
returns 201 + Location, matching the lastCreatedCard pattern already
used for card creation. A corresponding GET /steps/:stepId handler
serves the stored step on follow. Also adds a missing test for creating
a step with completed: true.

Closes #24

[greptile-apps]

Greptile Overview

Greptile Summary

This PR updates the MSW step-creation mock to better match the real Fizzy API by returning 201 Created with a Location header and no JSON body, and adds a corresponding GET /steps/:stepId handler to serve the created step on follow. It also adds a test case to cover creating a step with completed: true, ensuring the tool sends the correct request payload and parses the followed GET response.

Confidence Score: 4/5

• This PR is likely safe to merge, with one mock URL detail that may cause flaky/mismatched follow requests depending on client URL handling.
• Changes are localized to test mocks and tests, and align behavior with the described real API. Main risk is that the new relative Location header uses params.accountSlug (which may include a leading slash) and could generate a double-slash path or mismatch the client’s follow logic, causing tests to fail or leaving the follow path unexercised.
• src/test/mocks/handlers.ts (step POST Location header formatting)

[greptile-apps]

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