Make sure we check for executable's existence before submitting job

[shreyb]

Closes #631

This PR upgrades the executable argument in the jobsub get_parser parser to use a custom Action that does two things:

1. Check that the executable argument is given with the correct URI (file://)
2. Check that the argument value for the executable actually exists.

There are added accompanying unit tests, as well as a couple of updates to old tests so that they work with this new restriction.

All unit tests pass, and further testing information is available in comments of #631.

[Copilot]

Pull request overview

This PR enhances the executable argument validation in jobsub by replacing the simple type validation function with a comprehensive custom argparse Action class. The new implementation checks both that the executable argument uses the correct file:// URI format and that the file actually exists before job submission.

Key Changes

• Replaced verify_executable_starts_with_file_colon function with CheckExecutable custom action class that performs both URI format and file existence validation
• Updated the executable argument definition to use the new action with type=str and action=CheckExecutable
• Added three new unit tests covering valid executables, invalid URI format, and non-existent files
• Updated two existing tests to provide valid executable arguments now that validation is enforced

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
lib/get_parser.py	Removed old validation function and added CheckExecutable action class that validates URI format and file existence; updated executable argument to use the new action
tests/test_get_parser_unit.py	Added fixture and three tests for executable validation; updated existing tests to pass valid executable paths that comply with new validation

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[vitodb]

This looks good