fix(gerrit): defer change creation to createPr()

[felipecrs]

Changes

Gerrit has a very different workflow compared to other platforms. There is simply no concept of branches for the changes, just the changes themselves.

This PR changes the Gerrit platform implementation to defer the creation of the change to the createPr() method, rather than letting the change be created while preparing the branch.

This is better because it lets most of the standard Renovate platform flow continue as usual, which closes a few gaps:

• Closes [Gerrit] Reviewers are no longer (tried) to be assigned #39130
• Closes `prConcurrentLimit` does not do anything for Gerrit #39152
• Onboarding and config migration changes had the wrong subject in Gerrit
• Potentially fixes The `prHourlyLimit` counter for Gerrit gets incremented even when change is not actually created #35367

This PR also refactors part of #35900 and reverts #38288, as these hacks outside of the Gerrit platform can now be dropped.

Finally, this PR also changes Gerrit SCM deleteBranch() to delete the local branch as opposed to not deleting anything (the default SCM deletes both remote and local branches).

Context

Please select one of the below:

• This closes an existing issue: see above.
• This doesn't close an Issue, but I accept the risk that this PR may be closed if maintainers disagree with its opening or implementation

AI assistance disclosure

Did you use AI tools to create any part of this pull request?

Please select one option and, if yes, briefly describe how AI was used (e.g., code, tests, docs) and which tool(s) you used.

• No — I did not use AI for this contribution.
• Yes — minimal assistance (e.g., IDE autocomplete, small code completions, grammar fixes).
• Yes — substantive assistance (AI generated non‑trivial portions of code, tests, or documentation).
• Yes — other (please describe):

Claude Opus 4.6 was used to investigate places that needed updating, implement tests and review the code changes.

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

The public repository: https://review.gerrithub.io/q/project:felipecrs/renovate-repro-gerrit-defer-change-creation

[hanson76]

Is this being worked on, we have been forced to pin the version of Renovate to 41.167.0 which is the version prior to the change where Renovate stopped adding reviewers.

Looks like this pr has multiple parts, are all necessary to fix that bug or could it be split into several PRs that could be merged individually?

[felipecrs]

I'll look into completing this PR tomorrow.

[felipecrs]

Sorry, change of plans. I'll look into completing this next week or the other.

[avm99963]

Hey! As requested, I just took a look at the change.

I left some comments that popped up, but both the approach and the entire diff LGTM!

I also thought about an alternative approach while reading your PRs (but I think it is worse than yours):

Renovate's generic action	This PR's approach	An alternative approach
Create a branch	Create a branch locally without pushing	Create a WIP change
Create a PR	Submit a new change to Gerrit	Update the change to be active

It might be helpful to keep this alternative approach in mind in the case that, in the future, Renovate changes in some way such that it requires the branch to exist in the remote after branch creation. Since I don't think this is the case currently, your approach looks much simpler and cleaner!

[RahulGautamSingh]

ready for review? @felipecrs

[felipecrs]

Yes, absolutely. 😊