build(coordinator): go 1.25 (prometheus deps require it), bump Docker base

The metrics deps pull in modules whose go directive is 1.25, so go mod tidy
raised the module to go 1.25.0. The build image is bumped golang:1.24 ->
golang:1.25-alpine to match (the image runs GOTOOLCHAIN=local and can't
auto-fetch a newer toolchain). CI reads go-version-file, so it follows along.
client_golang pinned to v1.19.1 (stable, same API).

coordinator/Dockerfile

@@ -5,7 +5,7 @@
 # build fails with "the --mount option requires BuildKit".
 
 # --- build stage ----------------------------------------------------------
-FROM golang:1.24-alpine AS build
+FROM golang:1.25-alpine AS build
 
 WORKDIR /src