Test that rbs collection clean removes unneeded directories

`rbs collection clean` built a `Collection::Cleaner` and never called
`#clean` on it, so the command walked away with exit status 0 while
leaving every directory in place. The bug dates back to 38fd8b2, the
commit that introduced `rbs collection`, so the command had never once
removed anything.

`test/rbs/collection/cleaner_test.rb` exercises `Cleaner#clean`
directly, which is why the missing call went unnoticed for so long.
This test goes through `RBS::CLI#run` instead, so it covers the wiring
between the subcommand and the cleaner.

The lockfile lists `ast 2.4`. The collection directory also holds
`ast 2.3` and `rainbow 3.0`, covering both reasons `Cleaner#needed?`
rejects a directory: a gem locked at another version, and a gem absent
from the lockfile. `ast/2.4/ast.rbs` is written so that the assertion
on the version that survives shows its RBS files are left alone rather
than only that a directory still exists.

The collection is named with `--collection` rather than by changing the
working directory. `Config.find_config_path` walks from the working
directory up to the filesystem root, so a `Dir.chdir` based test stays
inside its temporary directory only as long as the config file is
written before `cli.run`. With `TMPDIR` pointed inside a real project, a
reordering would let the search reach an ancestor's lockfile and delete
that project's `.gem_rbs_collection`. `--collection` expands the path it
is given and skips the search.

The config file is written empty. `clean` reads `path` and `gems` from
the lockfile and never opens the config, and `path` appears in both
files, so giving the config a `path` of its own would suggest that the
command consults it. Its one remaining job is to be what `--collection`
names and what the lockfile path is derived from, which is why the
lockfile is written through `Config.to_lockfile_path` instead of a
hardcoded name that happens to match.

The `source` block in the lockfile stays even though `Cleaner#needed?`
does not read it. `Lockfile.from_lockfile` passes `gem["source"]` to
`Sources.from_config_entry`, so a lockfile without it raises
`NoMethodError` when read through `Lockfile` rather than `Config`.

#3127

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HSq3kKMB8ULhZrnL5urD7y

Author: pocke

test/rbs/cli_test.rb

@@ -1734,6 +1734,42 @@ def test_collection_install__set_pathname__manifest
     end
   end
 
+  def test_collection_clean
+    Dir.mktmpdir do |dir|
+      dir = Pathname(dir)
+
+      config_path = dir + RBS::Collection::Config::PATH
+      config_path.write("")
+
+      RBS::Collection::Config.to_lockfile_path(config_path).write(<<~YAML)
+        path: .gem_rbs_collection
+        gems:
+          - name: ast
+            version: "2.4"
+            source:
+              type: git
+              name: ruby/gem_rbs_collection
+              remote: https://github.com/ruby/gem_rbs_collection.git
+              revision: b4d3b346d9657543099a35a1fd20347e75b8c523
+              repo_dir: gems
+      YAML
+
+      collection_dir = dir + ".gem_rbs_collection"
+      (collection_dir + "ast/2.4").mkpath
+      (collection_dir + "ast/2.4/ast.rbs").write("class Ast end")
+      (collection_dir + "ast/2.3").mkpath
+      (collection_dir + "rainbow/3.0").mkpath
+
+      with_cli do |cli|
+        assert_cli_success cli.run(["--collection", config_path.to_s, "collection", "clean"])
+      end
+
+      assert_predicate(collection_dir + "ast/2.4/ast.rbs", :file?)
+      refute_predicate(collection_dir + "ast/2.3", :exist?)
+      refute_predicate(collection_dir + "rainbow/3.0", :exist?)
+    end
+  end
+
   def test_subtract
     Dir.mktmpdir do |dir|
       dir = Pathname(dir)