Scala: answer Metals' build-import prompt (#1769)

* scala: answer Metals' build-import prompt

Metals asks, via window/showMessageRequest, whether to import a workspace it
has not seen before. Serena registered no handler, so the request came back
`method 'window/showMessageRequest' not handled on client` and Metals logged
"Unexpected error initializing server" and gave up. No build server, no build
target, and every cross-file query answered by the fallback presentation
compiler — which cannot see past the file it is given.

A client cannot decline the question instead: Metals' `disableShowMessageRequest`
is server-side configuration, and its no-op fallback answers "Not now", which
imports nothing either. Answering is the only route to a build.

So answer the three prompts that lead to a build server, and dismiss anything
else with `null` — a prompt we do not recognise is one whose consequences we
cannot judge, and two of Metals' others offer to kill a process and to open a
window. "Don't show again" is never chosen; Metals persists that in the
project's own state.

Since answering yes lets Metals run the project's build tool,
`ls_specific_settings.scala.auto_import_build: false` declines instead.

* scala: name the build-tool choice as a gap, and test the setting

`Messages.ChooseBuildTool` ("Multiple build definitions found. Which would
you like to use?") offers the build tools' own executable names, and precedes
the import prompt wherever a workspace holds more than one kind of build. It
is dismissed like anything else unrecognised, so such a workspace is still
not imported — a deliberate choice, since picking one is a guess of a
different order, but one the comment and the setup guide should admit to
rather than claim every prompt on the path is answered.

Also: cover the route from `ls_specific_settings` to `auto_import_build`,
which nothing exercised, and correct the `ImportBuildChanges` message in the
fixtures, which had the notification variant's trailing full stop rather than
the request's own text.

docs/01-about/020_programming-languages.md

@@ -147,7 +147,7 @@ Some languages require additional installations or setup steps, as noted.
 * **Rust**  
   (requires [rustup](https://rustup.rs/) - uses rust-analyzer from your toolchain)
 * **Scala**  
-  (requires some [manual setup](../03-special-guides/scala_setup_guide_for_serena); uses Metals LSP)
+  (uses Metals LSP, which imports the build on first use — see the [setup guide](../03-special-guides/scala_setup_guide_for_serena))
 * **SCSS / Sass / CSS**
   (experimental; requires Node.js + npm; uses [some-sass-language-server](https://github.com/wkillerud/some-sass) to handle
   `.scss`, `.sass`, and `.css`)

docs/03-special-guides/scala_setup_guide_for_serena.md

@@ -15,7 +15,16 @@ Install the following on your system and ensure they are available on `PATH`:
   - Serena uses `cs` if available; if only `coursier` exists, it will attempt to install `cs`. If neither is present, install Coursier first.
 
 ---
-## Quick Start (Recommended: VS Code + Metals auto‑import)
+## Quick Start
+
+Start Serena in your project root. Metals asks whether to import a workspace it has not seen before, and Serena answers that prompt for it — the build is imported (for sbt, by running `sbt bloopInstall`), `.bloop/` and `.metals/` are created, and cross-file navigation works from there. The first run therefore takes as long as your build takes to load.
+
+Set `auto_import_build: false` under `ls_specific_settings.scala` to decline instead; you then need to import the build yourself by one of the routes below, or cross-file queries will be served by the fallback presentation compiler and see only one file at a time.
+
+Serena answers three of Metals' prompts — “Import build”, “Import changes”, “Connect”. Anything else Metals asks is dismissed and logged, including “Multiple build definitions found. Which would you like to use?”, so a workspace holding more than one kind of build (say both an sbt and a Maven definition) still needs importing by one of the routes below.
+
+---
+## Importing the build yourself (VS Code)
 
 1. Open your Scala project in VS Code.
 2. When prompted by Metals, accept “Import build”. Wait until the import and initial compile/indexing finish.
@@ -25,7 +34,7 @@ Install the following on your system and ensure they are available on `PATH`:
 This flow ensures the `.bloop/` and (if applicable) `.metals/` directories are created and your build is known to the build server that Metals uses.
 
 ---
-## Manual Setup (No VS Code)
+## Importing the build yourself (No VS Code)
 
 Follow these steps if you prefer a manual setup or you are not using VS Code: