Persist the base commit of subscription update for diff links

[adamzip]

#4777 (comment)

Persist the original base commit throughout subsequent subscription updates on the same open PR in order to generate the right commit diff url.

Example PR: maestro-auth-test/maestro-test-vmr#1850

[Copilot]

Pull Request Overview

This PR ensures that the original base commit SHA for a subscription is persisted across multiple updates on the same pull request, enabling correct diff link generation.

• Adds a BaseSourceSha property to the subscription update model.
• Populates BaseSourceSha when creating and updating code‐flow pull requests.
• Passes BaseSourceSha into the PR title/description builder for accurate diff URLs.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/ProductConstructionService/.../SubscriptionPullRequestUpdate.cs	Added BaseSourceSha DataMember to track original commit
src/ProductConstructionService/.../PullRequestUpdater.cs	Computes and assigns BaseSourceSha in create/update flows

Comments suppressed due to low confidence (1)

src/ProductConstructionService/ProductConstructionService.DependencyFlow/PullRequestUpdater.cs:1114

• Consider adding unit tests to verify that BaseSourceSha is correctly retrieved for existing subscriptions and falls back to previousSourceSha when absent.

string? baseSourceSha= pullRequest.ContainedSubscriptions.FirstOrDefault(u => u.SubscriptionId.Equals(update.SubscriptionId))?.BaseSourceSha

[Copilot]

[nitpick] Add XML documentation for BaseSourceSha explaining that it holds the original base commit SHA used for generating diffs.

Suggested change

	/// <summary>
	/// Gets or sets the original base commit SHA used for generating diffs.
	/// </summary>

[premun]

This would be nice to add

[adamzip]

It's a bit difficult to describe. I'm thinking "The earliest commit in the source repository from the changes contained in the current PR".
While writing this, I realize that this property is a bit out of place - technically, SubscriptionPullRequestUpdate shouldn't depend on any PR state - it only represents a single subscription update (i.e. changes from one subscription trigger), while BaseSourceSha only makes sense in the context of the PR

[premun]

That's a good point. Why isn't it in the InProgressPullRequest?

[adamzip]

I'll change it