CHANGELOG.md: add release heading for v0.51.1

This is an automated CL which updates the CHANGELOG.md.

Change-Id: I35957152dcc71b57b4752c09be5d715dfe98532c
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/715280
Reviewed-by: Madeline Kalil <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Hongxiang Jiang <[email protected]>
Auto-Submit: Gopher Robot <[email protected]>

Author: gopherbot

CHANGELOG.md

@@ -75,6 +75,15 @@ golang/vscode-go#3862).
 * Resolved a problem where `staticcheck` was being installed automatically even
 when it was not in use (golang/vscode-go#3898).
 
+## v0.51.1 (prerelease)
+
+Date: 2025-10-27
+
+This is the [pre-release version](https://code.visualstudio.com/api/working-with-extensions/publishing-extension#prerelease-extensions) of v0.52.
+
+**Full Changelog**: https://github.com/golang/vscode-go/compare/v0.50.0-rc.1...v0.51.1
+**Milestone**: https://github.com/golang/vscode-go/issues?q=milestone%3Av0.52.0
+
 ## v0.51.0 (prerelease)
 
 Date: 2025-09-04