Add unit tests for mod endpoints

[pavog]

This PR adds unit tests for the mod endpoints of CurseForgeAPIClient. They run against the mocked HTTP client from ClientTestCase.

searchMods()

• A search for “mclogs” with pagination, with the results mapped to Mod
• All options are set and every query parameter is checked. categoryIds, gameVersions and modLoaderTypes are sent as JSON arrays, and clientCompatible as true (see Fix boolean query parameters #14).
• The page size is capped at 50, and at 10 when the offset is 9990, so the request stays within the API’s 10,000-result limit.

getMod()

• getMods(): once with default values, once with devModIds and filterPcOnly
• getFeaturedMods(): once with default values, once with excludedModIds, gameVersionTypeId and clientCompatible
• getModDescription(): once without lang, once with it

also

ClientTestCase gets a new helper, assertPostRequest(), which checks the method, the path and the JSON body of a request. The tests record the current behaviour that null values are sent explicitly in the body, for example "filterPcOnly": null.

The fixtures are recorded responses from the real CurseForge API. To keep them small, latestFiles and latestFilesIndexes are cut down to a few entries per mod.