Fix inaccurate exception message in TaskExecutorJobLauncher

[kojw1017]

The JobExecutionAlreadyRunningException thrown by TaskExecutorJobLauncher currently includes the JobInstance details instead of the running JobExecution details.

This commit updates the exception to pass the running JobExecution.

Resolves #5280

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[kojw1017]

I've pushed a unit test covering this change, since CONTRIBUTING.md asks for JUnit coverage on behavior changes and the original commit had none. TaskExecutorJobLauncherTests didn't exist yet, so it's a new file following the style of the neighbouring CommandLineJobOperatorTests. Reverting the one-line fix makes it fail, so it does pin the behaviour rather than just passing.

Two things that may be worth knowing while reviewing:

• Although TaskExecutorJobLauncher is deprecated for removal, TaskExecutorJobOperator inherits createJobExecution() through SimpleJobOperator and the method is private, so this fix also corrects the message produced by the currently recommended API. I verified that by driving the same scenario through TaskExecutorJobOperator.start().
• Line 175 of the same class already passes lastJobExecution, so this brings line 137 in line with the existing form in the file.

The branch is based on the original point rather than rebased, since the PR still reports as mergeable — happy to rebase if you'd prefer. Also glad to open the same change against 6.0.x if the 6.0.6 milestone needs it; just let me know which you'd rather have.