[monitorlib] Handle gracefully missing field in versioning client

[the-glu]

Fix #1311

Check for field presence and raise a VersionQueryError if not present.
Add some unit tests. NB: Those test are not run in the code base. #1313 fixes that.

Also tested locally against a 'rogue' mockuss / f3548_self_contained config:

Before:

 ERROR    | monitoring.uss_qualifier.suites.suite:_run_test_scenario:181 - Execution error:
  Traceback (most recent call last):
    File "/venv/lib/python3.13/site-packages/implicitdict/__init__.py", line 156, in __getattribute__
      return self[item]
             ~~~~^^^^^^
  KeyError: 'system_identity'
  
  During handling of the above exception, another exception occurred:
  
  Traceback (most recent call last):
    File "/app/monitoring/uss_qualifier/suites/suite.py", line 163, in _run_test_scenario
      scenario.run(context)
      ~~~~~~~~~~~~^^^^^^^^^
    File "/app/monitoring/uss_qualifier/scenarios/versioning/get_system_versions.py", line 30, in run
      resp = version_provider.get_version(self._system_identity)
    File "/app/monitoring/monitorlib/clients/versioning/client_interuss.py", line 46, in get_version
      if resp.system_identity != version_type:
         ^^^^^^^^^^^^^^^^^^^^
    File "/venv/lib/python3.13/site-packages/implicitdict/__init__.py", line 158, in __getattribute__
      raise AttributeError
  AttributeError

After:

WARNING  | monitoring.uss_qualifier.suites.suite:_print_failed_check:71 - New failed check:
  details: 'Response to get version didn''t return a system identity
  
    Severity Severity.High upgraded to Critical because `stop_fast` flag set true in
    configuration'
  documentation_url: https://github.com/interuss/monitoring/blob/b1322faf3c107d3970efc6ed639176fe50ee0fb8/monitoring/uss_qualifier/scenarios/versioning/get_system_versions.md#valid-response-check
  name: Valid response
  participants:
  - uss1_core
  query_report_timestamps: []
  requirements:
  - versioning.ReportSystemVersion
  severity: Critical
  summary: Error querying version