CHANGELOG.md: add release heading for v0.49.0

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

Change-Id: I7eccb56eb23777f4086c384e79ed3a3799beff43
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/685975
kokoro-CI: kokoro <noreply+kokoro@google.com>
Reviewed-by: Hongxiang Jiang <hxjiang@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Auto-Submit: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: gopherbot

CHANGELOG.md

@@ -5,6 +5,15 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/).
 
 ## Unreleased
 
+## v0.49.0 (prerelease)
+
+Date: 2025-07-07
+
+This is the [pre-release version](https://code.visualstudio.com/api/working-with-extensions/publishing-extension#prerelease-extensions) of v0.50.
+
+**Full Changelog**: https://github.com/golang/vscode-go/compare/v0.48.0-rc.1...v0.49.0
+**Milestone**: https://github.com/golang/vscode-go/issues?q=milestone%3Av0.50.0
+
 ## v0.48.0
 
 Date: 2025-06-30