build: bump Go toolchain to 1.25.9 (#24293)

Bumps the Go toolchain from 1.25.8 to 1.25.9 across `go.mod`, the shared
setup-go action, and the dogfood image.
This keeps local builds, CI, and containerized workflows aligned on the
latest patch release, including the updated Go tarball checksum in the
Dockerfile.

.github/actions/setup-go/action.yaml

@@ -4,7 +4,7 @@ description: |
 inputs:
   version:
     description: "The Go version to use."
-    default: "1.25.8"
+    default: "1.25.9"
   use-cache:
     description: "Whether to use the cache."
     default: "true"