Theoretically the max pages would be 65536 (2byte unsigned int.
However this limit is much higher than practical documents have
and larger ones can lead to unforseen problems, for example RAM
limitations.
We thus opted to use a lower limit of 10K. The limit must be
detected client-side, given that the server is distrusted. However
we also check it in the server, just as a fail-early mechanism.
Isolation provider tests done in tests/test_base.py and had
pytest.mark.parameterize() for each isolation provider. This logic
would not work well when we had test that diverge. We could have marked
each one as compatible with one provider or another, but in the end it
turned out to be better to have the common ones in a base class and
the divergent ones in each.
NOTE: this has a strange side-effect: inherited test classes need to
have imports for all of the fixtures even if they are not explictly used
Improve the `parse_progress()` method of the container isolation
provider in the following ways:
1. Make sure that the fields of the progress report have the expected
type.
2. In case of a JSON parsing error, sanitize the invalid string so that
it doesn't contain escape sequences, or the user considers it as
trusted.
Update the common `print_progress()` method in the base
`IsolationProvider` class, with two extra features:
1. Always sanitize the provided text argument.
2. Mark the sanitized text argument as untrusted.
This is default behavior from now on, since this function is commonly
used to parse progress reports from the conversion sandbox.
