feat: [PIPE-29374]: make author/committer fields optional for GitHub App signed commits

[ritek01]

Make author/committer fields optional for GitHub App signed commits

Overview

This PR modifies the GitHub driver in go-scm to make author and committer fields optional when using GitHub App authentication for commits. This change enables automatic commit signing by GitHub Apps without requiring explicit signature information.

Problem

Ticket : https://harness.atlassian.net/browse/PIPE-29374

When using GitHub App authentication, commits can be automatically signed by GitHub without providing explicit author/committer information. However, the current implementation in go-scm always includes these fields in the request payload, which prevents GitHub from automatically signing the commits.

Solution

This PR modifies the content service in the GitHub driver to:

1. Make author/committer fields optional in the request payload
2. Only include these fields when signature information is explicitly provided
3. Omit these fields when using GitHub App authentication, allowing GitHub to automatically sign the commits

Changes

• Updated contentService.Create() to omit author/committer fields when signature is empty
• Updated contentService.Update() to omit author/committer fields when signature is empty
• Updated contentService.Delete() to omit author/committer fields when signature is empty
• Added comments explaining the GitHub App signed commits behavior

Testing

• Verified with a proof of concept that commits are properly signed when using GitHub App authentication
• Confirmed backward compatibility with existing code that provides explicit signature information
• Tested with real GitHub App credentials to ensure commits show as "Verified" in GitHub UI

Screenshot

Screen.Recording.2025-08-21.at.7.09.10.PM.mov

[CLAassistant]

All committers have signed the CLA.

[Copilot]

Pull Request Overview

This PR enables automatic commit signing for GitHub Apps by making author and committer fields optional in the GitHub driver's content service. This allows GitHub to automatically sign commits when using GitHub App authentication instead of requiring explicit signature information.

Key changes:

• Modified content service methods (Create, Update, Delete) to conditionally include author/committer fields
• Added logic to omit author/committer when signature name/email are empty
• Updated struct field types to use pointers with omitempty JSON tags

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