refactor: isolate EditTask logic without changing behavior

[Harshkamboj11]

Description

This PR refactors the EditTask flow by isolating the internal logic into a
request-based implementation while preserving the existing public API.

Scope of this PR is intentionally limited to EditTask only.

What was done

• Refactored EditTask logic into a single internal function
• Preserved the existing EditTaskInTaskwarrior function signature
• No changes to AddTask, Sync, Export, or other Taskwarrior flows
• No changes to existing taskwarrior tests

What was NOT changed

• AddTask-related logic
• Existing tests in taskwarrior_test.go
• API contracts or request/response formats

Notes

This refactor is behavior-preserving and focuses only on improving
internal structure and maintainability of EditTask.

[github-actions]

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