Rename the DOCKER_IN_MEMORY test backend to FLASK_IN_PROCESS

[adamtheturtle]

Fixes #3380.

The DOCKER_IN_MEMORY backend mounts the three Flask applications of the mock into one responses mock in the test process. That exercises the Flask handlers which the Docker deployment runs, but not the split between its containers. Its name and its test ID, "In Memory version of Docker application", read as though it covered the Docker deployment, which #3380 identified as misleading.

The second half of #3380, extending test_docker.py to cover the stateful features which depend on the container split, has already landed: that file now has round trips for Model Target datasets, reco counts reports, VuMark instance generation, rate limiting and database deletion. This is the remaining part, the rename.

• VuforiaBackend.DOCKER_IN_MEMORY is now VuforiaBackend.FLASK_IN_PROCESS, with the test ID "In Process Flask Applications".
• The setup functions and the docstrings say what the backend covers and point to test_docker.py for the container split.
• pytest-multi-backend derives the option from the member name, so --skip-backend=docker_in_memory is now --skip-backend=flask_in_process. The CI workflow and the contributing docs are updated.

No news fragment: this only touches the test suite, the CI workflow and the contributing docs.

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