automergeStrategy not applied when using platformAutomerge on GitHub

[rcdailey]

Problem

When platformAutomerge: true (the default) and automergeStrategy: "rebase" are configured, GitHub platform automerge still uses the repo-level default merge method (typically squash) instead of rebase.

This is a follow-up to #16884, which was closed as resolved in v37.110.0. That fix wired automergeStrategy into the mergePr code path (Renovate-native merge), but the tryPrAutomerge / enableAutoMerge code path (used when platformAutomerge: true) was not updated.

Code

In lib/modules/platform/github/index.ts (line 1789), tryPrAutomerge uses only config.mergeMethod (auto-detected from repo settings) and ignores the automergeStrategy that is already available via platformPrOptions:

const mergeMethod = config.mergeMethod?.toUpperCase() || 'MERGE';

By contrast, mergePr (line 1958) correctly consults the strategy:

const mergeStrategy = mapMergeStartegy(strategy) ?? config.mergeMethod;

The PlatformPrOptions interface already carries automergeStrategy (types.ts line 103), and it is already passed into tryPrAutomerge (line 1761), but never read.

Suggested Fix

// line 1789, change from:
const mergeMethod = config.mergeMethod?.toUpperCase() || 'MERGE';

// to:
const mergeMethod =
  (mapMergeStartegy(platformPrOptions?.automergeStrategy) ?? config.mergeMethod)?.toUpperCase() || 'MERGE';

I'm happy to submit a PR for this if the approach looks correct.