This is a minimal refactor preparatory PR.

It changes the `utils` module from a file to a folder, in anticipation
of other PR that may add further utils - to avoid that everyone works on
the same file.

Adds documentation for setting up a developer environment, covering API
v1, API v2, and python SDK.

Adds link to developer setup from documentation page.

… `production_server` (openml#1632)

* registers `test_server` marker, fixes openml#1631.
* renames `production` marker to `production_server`

Update the tests to allow connecting to a local test server instead of a
remote one (requires openml/services#13).

Running the tests locally:
- Locally start the services (as defined in
openml/services#13) using `docker compose
--profile "rest-api" --profile "evaluation-engine" up -d`. Startup can
take a few minutes, as currently the PHP container still builds the ES
indices from scratch.
    
I noticed that the `start_period` for some services isn't sufficient on
my M1 Mac, possibly due to some containers requiring Rosetta to run,
slowing things down. You can recognize this by the services reporting
"Error" while the container remains running. To avoid this, you can
either increase the `start_period` of the services (mostly elastic
search and php api), or you can simply run the command again (the
services are then already in healthy state and the services that
depended on it can start successfully).
  
The following containers should run: openml-test-database,
openml-php-rest-api, openml-nginx, openml-evaluation-engine,
openml-elasticsearch, openml-minio

- Update the `openml/config.py`'s `TEST_SERVER_URL` variable to
`"http://localhost:8000"`.
 - Run the tests (`python -m pytest -m "not production" tests`).


This PR builds off unmerged PR
openml#1620.

---------

Co-authored-by: Armaghan Shakir <raoarmaghanshakir040@gmail.com>

…ing (openml#1570)

## Overview

This PR introduces a new **`OpenMLAuthenticationError`** exception to
clearly distinguish **authentication errors** (invalid or missing API
key) from **authorization errors** (valid API key without sufficient
permissions).

---

## Changes

### **New Exception**

* Added **`OpenMLAuthenticationError`** in `exceptions.py`
* Inherits from `OpenMLServerError` for consistency
* Automatically appends helpful guidance with links to:

* Getting an API key: [https://www.openml.org/](https://www.openml.org/)
  * OpenML authentication documentation
* Includes a clear docstring explaining the difference between
authentication and authorization errors

---

### **Updated Error Handling**

* Updated `_api_calls.py` to:

* Import and raise `OpenMLAuthenticationError` for authentication
failures

---

### **Tests Updated**

* Updated
`test_authentication_endpoints_requiring_api_key_show_relevant_help_link`

* Now expects `OpenMLAuthenticationError` instead of
`OpenMLNotAuthorizedError`
* Continues to assert that helpful guidance is included in the error
message

---

Fixes openml#1562

…ing ClassVar (openml#1588)

Fixes openml#1578