Fix error in ISIS Diagnostics tab of Indirect Reduction

[robertapplin]

Description of work

This PR fixes a bug on the ISIS Diagnostics tab of the Indirect Data Reduction interface where the incorrect load algorithm was being used to load a raw file. The assumption was that the LoadNexusProcessed algorithm should be used if the LoadHistory=true property was provided. Instead, we should rely on the Load algorithm to choose the correct loader, and then only set the LoadHistory property if this property exists on the chosen load algorithm.

Fixes #37808

No tests have been added because this part of the code is not in MVP.

To test:

1. Open Indirect Data Reduction
2. Select IRIS instrument
3. Go to ISIS Diagnostics tab
4. Type in 26176 and press enter
5. The data will load and a graph plotted in the embedded interface plot.
6. There should be NO red asterisk next to the File selector widget.

This does not require release notes because the bug was introduced after the last stable release of Mantid

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[warunawickramasingha]

Tested and working as explained! Happy to approve if the tests are passing!