feat(packer): restore packer-manifest.json state across blueprint regenerations

[kadupoornima]

Implement State Restoration for Packer Images (restoreState)

The Problem

The moduleWriter interface defines a restoreState method, which is intended to transfer state files from a previous deployment run when a user regenerates their blueprint (preventing state loss).

While TFWriter.restoreState is fully implemented and restores .tfstate and .tfstate.backup files, PackerWriter.restoreState is currently a no-op:

func (w PackerWriter) restoreState(deploymentDir string) error {
	// TODO: restore packer-manifest.json if it exists
	return nil
}

When a user modifies their blueprint and runs ghpc create again, the toolkit rewrites the deployment directories. Because Packer's state (packer-manifest.json, which tracks built image IDs) is not restored, the toolkit loses track of the images built in the previous run. This forces the toolkit to rebuild the Packer images from scratch, even if their configuration did not change.

The Solution

Implement PackerWriter.restoreState to copy the packer-manifest.json file from the backup directory (.ghpc/previous_system/ or equivalent) into the newly generated Packer deployment directory.

The Impact

• Developer Experience: Avoids extremely time-consuming, redundant VM image builds when iterating on blueprints.
• Efficiency: Saves compute resources and time by preserving the cache of already-built Packer images across blueprint updates.

[gemini-code-assist]

Summary of Changes

Hello, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request addresses an issue where Packer image state was lost during blueprint regeneration, causing unnecessary and time-consuming rebuilds. By implementing the restoreState logic for Packer, the toolkit now correctly identifies and preserves existing image manifests, significantly improving developer efficiency and resource usage.

Highlights

• State Restoration Implementation: Implemented the restoreState method in PackerWriter to copy packer-manifest.json from previous deployment directories to the current one.
• Efficiency Improvements: Prevents redundant Packer image builds by preserving the image manifest across blueprint regenerations.
• Testing: Added a new unit test TestRestorePackerState to verify that the manifest file is correctly located and copied during the restoration process.

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[gemini-code-assist]

Code Review

This pull request implements the restoration of packer-manifest.json files from previous deployment groups in PackerWriter.restoreState and adds a corresponding unit test. The review feedback points out that checking err == nil on os.Stat silently ignores unexpected system errors (like permission issues); it suggests explicitly checking for os.ErrNotExist to skip restoration and returning other errors.