Minor HTCondor run scripts improvements

[GernotMaier]

Minor updates:

• check that apptainer image actually exists (otherwise jobs will be in "H" state, and one has to find the log files for debugging)
• replace test_layout in integration test file (which does not exist anymore)

[Copilot]

Pull request overview

This PR improves the HTCondor submission script generation workflow by adding a pre-flight check for the configured Apptainer image file and updating an integration-test configuration to use a version-dependent array layout name.

Changes:

• Add an existence check for the configured Apptainer image before generating HTCondor submit/run scripts.
• Extend unit tests to cover the new “missing image” failure mode.
• Update an integration-test config to use array_layout_name.by_version instead of the removed test_layout.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 3 comments.

File	Description
src/simtools/job_execution/htcondor_script_generator.py	Validate apptainer_image exists and pass a Path through to submit-file generation.
tests/unit_tests/job_execution/test_htcondor_script_generator.py	Add coverage for the new image existence check (success + failure cases).
tests/integration_tests/config/simulate_prod_htcondor_generator_gamma_20_deg_north.yml	Replace obsolete array_layout_name with a version-dependent mapping.

[ctao-sonarqube]

Quality Gate passed

Issues
0 New issues
0 Fixed issues
0 Accepted issues

Measures
0 Security Hotspots
83.3% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube