Fix formatter _build dir lookup when inside an umbrella app

[JesseHerrick]

Note

Medium Risk
Changes how the persistent formatter locates _build and deps and how arguments are passed into the Elixir formatter script, which can affect formatting/plugin loading across projects (especially umbrella layouts). Includes a new integration-style test covering symlinked _build behavior, reducing regression risk but still touching core formatting flow.

Overview
Fixes persistent formatter behavior inside umbrella Elixir apps by teaching formatter_server.exs to walk up from mix_root (bounded by projectRoot) to find the correct ancestor containing _build, and then using that project_root for plugin code paths and import_deps .formatter.exs resolution.

Updates the Go launcher (formatter.go) to pass projectRoot into the formatter script, and adds TestFormatterServer_UmbrellaStylerPlugin to verify Styler plugins load and apply when _build exists only at the umbrella root.

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