refactor: tests

[pd93]

Description

This PR attempts to tidy up the ever increasing task_test.go file and its various testing methodologies. This work was started a long time ago but stalled due to the vast volume of tests that needed migrating.

To reduce the immense amount of time this migration would take by hand, this PR was largely written using Claude Sonet 5. The agent was given the following goals across several independent prompt:

• Move all tests out of task_test.go. The end result should contain helper functions, setup and functional options boilerplate only. No actual functional tests.
• Tests that belong to the Executor should use NewExecutorTest where possible. Anything that uses this setup will generate a fixture file with the expected output of that test. Tests that are not appropriate to this style of test may use another approach when necessary.
• Remove fileContestTest entirely
• Functional options should return concrete types and all be in located in task_test.go

This PR will remain in draft until I fully review all the tests to ensure that nothing has been clobbered. All the tests pass, but they need checking to ensure they still test the Executor correctly.

Checklist

• I have read and followed the Contribution Guide.
• I have disclosed the use of any AI-generated content in this pull request per the AI Usage Policy.
• I fully understand the changes and have hand-written the description (No AI) of this pull request.