refactor: Small improvements to the 'On Completion' implementation

[claremacrae]

Types of changes

Internal changes:

• Refactor (prefix: refactor - non-breaking change which only improves the design or structure of existing code, and making no changes to its external behaviour)
• Tests (prefix: test - additions and improvements to unit tests and the smoke tests)

Description

• Refactor the On Completion code to make wider use of the OnCompletion enum.
• Move the code for parsing On Completion values from DefaultTaskSerializer.ts to OnCompletion.ts so that all edits for new or changed options are in the same file.
• Add new test to ensure that the EditTask modal retains any OnCompletion value (it cannot yet edit the value)

Motivation and Context

Getting ready to release the work done in:

• feat: New "On Completion" field with support for Delete automatic action #2840

How has this been tested?

• Automated tests
• Exploratory testing

Checklist

• My code follows the code style of this project and passes yarn run lint.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• My change has adequate Unit Test coverage.

Terms

• My contribution follows this project's contributing guide
• I agree to follow this project's Code of Conduct

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

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

See analysis details on SonarCloud