Add unit tests for executeApplyStage, executePlanStage, and executeRo…

[vishnukothakapu]

What this PR does: Adds unit tests for three stage-execution functions in the pipedv1 Terraform plugin: executeApplyStage, executePlanStage, and executeRollbackStage. It also removes the // TODO: add test comments from their respective source files.

The new test suites cover the most common pre-execution failure scenarios (such as an unavailable StageLogPersister across all stages, and an empty CommitHash during the first deployment rollback).

Why we need it: These functions are core to the Terraform plugin's deployment lifecycle but were entirely untested. Adding tests here improves confidence in the pipedv1 plugin architecture, resolves long-standing TODOs, and aligns with the project's existing testing patterns seen in plugin_test.go.

Which issue(s) this PR fixes: Fixes #7108

Does this PR introduce a user-facing change?:

• How are users affected by this change: No, this is an internal test addition only.
• Is this breaking change: No
• How to migrate (if breaking change): N/A

[github-actions]

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