build: Add ldmiddleware to "make bump-min-go-version"

[aaron-zeisler]

Summary

Adds ldmiddleware to the bump-min-go-version Makefile target and the CONTRIBUTING docs.

Background

The repo has five Go modules — go.mod, ldotel/go.mod, ldai/go.mod, ldmiddleware/go.mod, and testservice/go.mod — all currently on go 1.24.0. The bump-min-go-version target is meant to move the minimum version across all of them (plus min= in .github/variables/go-versions.env) in one shot.

ldmiddleware was added after the target was written and never wired in. Both the Makefile target and the CONTRIBUTING file that documents it omitted it. The result is a latent drift bug: the next time someone runs make bump-min-go-version MIN_GO_VERSION=1.25, the other four modules and the env file would advance while ldmiddleware silently stayed behind. It only lines up today because it was set manually.

Changes

• Makefile — add the cd ldmiddleware && go mod edit -go=$(MIN_GO_VERSION) go.mod line alongside the other modules.
• CONTRIBUTING.md — add ldmiddleware/go.mod to the documented list of files the command updates.

No version value changes; all five modules remain at go 1.24.0. Verified by running make bump-min-go-version MIN_GO_VERSION=1.24.0 (a no-op re-set) and confirming no unexpected diff.

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Note

Low Risk
Documentation and Makefile-only change to a maintainer tooling target; no runtime or dependency version changes.

Overview
Wires the ldmiddleware Go module into the shared minimum-Go bump workflow so it stays aligned with the other modules.

make bump-min-go-version now runs go mod edit in ldmiddleware alongside go.mod, ldotel, ldai, and testservice, and CONTRIBUTING.md lists ldmiddleware/go.mod among the files that command updates. No Go version values change in this PR.

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