Align and complete test-suite coverage across endpoint modules

### Background

Following the discussion in #196: the endpoint coverage has grown a fair bit recently, and the test suite would benefit from a pass to make coverage consistent across all modules.

The suite already combines two complementary approaches, and the idea here is to apply them uniformly rather than change them:

- **Live integration tests** against the Fossology test instance in CI (`fossologytests.yml`, `fossology/fossology:4.4.0`)
- **Mocked-request tests** (`responses`) for the API error paths

### Proposal

For each endpoint wrapper, aim for:

1. one live happy-path test validating the basic request/response against the CI Fossology instance, and
2. mocked tests for the documented error responses (e.g. `403` → `AuthorizationError`, `404`, `500`).

### Suggested steps

- [ ] Audit the endpoint modules (`folders`, `uploads`, `jobs`, `items`, `license`, `report`, `search`, `groups`, `users`) and list where a live happy-path test or a mocked error test is missing.
- [ ] Fill the gaps, following the existing patterns (`foss` fixture for live paths, `@responses.activate` for errors).
- [ ] Note the coverage convention briefly in the docs so new endpoints follow it.

### Non-goals

- No change to the CI setup or the pinned Fossology version.
- No new endpoints here (those stay in separate PRs).

I'm happy to take this on and split it into small per-module PRs if that's easier to review. Does this match what you had in mind, @deveaud-m?

Start by auditing the endpoint modules named in the issue—folders, uploads, jobs, items, license, report, search, groups, and users—against their existing tests. Follow the `foss` fixture for live paths and `@responses.activate` for mocked errors, then check `fossologytests.yml` for the CI instance details. Done means each wrapper has the requested happy-path and error coverage, with the convention noted in the docs.

Written by the indexing model from the issue text.