[LocalLiveView] Generator tests

[FranekKubis]

Summary

• Tests for mix llv.install (test/mix/tasks/llv_install_test.exs) — one test per inject_* step + generate_local_project + idempotency, using Igniter.Test.phx_test_project().
• mix test step added to .github/workflows/local_live_view_build.yml.

Side effect: a few inject_* steps used real-FS path checks and silently no-op'd outside the target project's cwd — refactored to Igniter's virtual-FS APIs.

Test plan

• CI green on the LocalLiveView workflow
• mix llv.install still works on a fresh phx.new project

[mat-hek]

Let's add an integration test, that runs mix phx.new, adds llv to deps, updates home.html.heex, runs llv.install, mix setup and mix phx.server, visits the page with playwright and checks if HelloLocal renders properly