Tests: Verbindungspools nur der eigenen Datenbank leeren statt aller

Die zufaelligen roten Laeufe der vollen Suite (jedes Mal ein anderer Test,
zuletzt PromptHistoryTests mit einer reinen Funktion) hatten eine Ursache:
Zwanzig Testklassen riefen beim Aufraeumen SqliteConnection.ClearAllPools()
auf. Das ist prozessweit und nahm einem parallel laufenden Test die
Verbindung mitten im Oeffnen weg - ObjectDisposedException in ApplyPragmas.

- SqliteStorage.ReleasePooledConnections(pfad) leert nur die Pools der
  Datenbanken unter einer Datei oder einem Verzeichnis.
- BackupService oeffnet die Quelle fuer VACUUM INTO ohne Pool: eine
  einmalige Kopie soll die Datei danach nicht offen halten.
- PoolReleaseTests: die freigegebene Datenbank laesst sich loeschen, die
  andere arbeitet weiter.

Vier volle Laeufe hintereinander gruen.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/ClawdDotNet.Core/Storage/SqliteStorage.cs

@@ -1,3 +1,4 @@
+using System.Collections.Concurrent;
 using Microsoft.Data.Sqlite;
 
 namespace ClawdDotNet.Core.Storage;
@@ -19,6 +20,9 @@ public sealed class SqliteStorage
     private readonly string _connectionString;
     private readonly SemaphoreSlim _writeGate = new(1, 1);
 
+    /// <summary>Every database opened in this process: full path → connection string.</summary>
+    private static readonly ConcurrentDictionary<string, string> OpenedDatabases = new(StringComparer.OrdinalIgnoreCase);
+
     public string DatabasePath { get; }
 
     public SqliteStorage(string databasePath)
@@ -37,6 +41,7 @@ public sealed class SqliteStorage
             Pooling = true,
             DefaultTimeout = 30
         }.ToString();
+        OpenedDatabases[Path.GetFullPath(databasePath)] = _connectionString;
 
         Initialize();
 
@@ -51,6 +56,33 @@ public sealed class SqliteStorage
         }
     }
 
+    /// <summary>
+    /// Closes the pooled connections of the databases at <paramref name="path"/> — a
+    /// database file, or a directory and everything below it — so the files can be deleted
+    /// or replaced.
+    ///
+    /// <para>Not <see cref="SqliteConnection.ClearAllPools"/>: that is process-wide and also
+    /// takes the connections of every other database in use. The tests used it to clean up
+    /// their temporary databases, and a test running in parallel then lost its connection
+    /// while opening it — an ObjectDisposedException in <c>ApplyPragmas</c>, one in every
+    /// few full runs, always in a different test.</para>
+    /// </summary>
+    public static void ReleasePooledConnections(string path)
+    {
+        var full = Path.GetFullPath(path);
+        var below = full.TrimEnd(Path.DirectorySeparatorChar, Path.AltDirectorySeparatorChar) + Path.DirectorySeparatorChar;
+
+        foreach (var (database, connectionString) in OpenedDatabases)
+        {
+            if (!string.Equals(database, full, StringComparison.OrdinalIgnoreCase)
+                && !database.StartsWith(below, StringComparison.OrdinalIgnoreCase))
+                continue;
+
+            using var connection = new SqliteConnection(connectionString);
+            SqliteConnection.ClearPool(connection);
+        }
+    }
+
     public SqliteConnection OpenConnection()
     {
         var connection = new SqliteConnection(_connectionString);