Every credential provider says whether it still holds a consumer

holds() for postgres, mssql, mongodb, minio, lavinmq, mosquitto, mailu
and gitea, so the harness makes again a login the backend lost (hq issue
120). Each checks the mesh's password as the consumer presents it, or
compares it read-only, and returns false only when the backend says the
credential is absent or wrong; an unreachable backend throws.

modules/mongodb/client.ts

@@ -109,6 +109,32 @@ print(EJSON.stringify({ ok: 1 }));
     await this.evalJs<{ ok: number }>(js);
   }
 
+  /**
+   * Whether `user` authenticates against `database` with exactly `password` and holds `dbOwner`
+   * there: checked by connecting as the consumer, the way it connects. Read-only. `false` only on an
+   * authentication failure or a missing role; an unreachable server rejects (novox/hq issue 120).
+   */
+  async canAuthenticateAs(database: string, user: string, password: string): Promise<boolean> {
+    const uri =
+      `mongodb://${encodeURIComponent(user)}:${encodeURIComponent(password)}@${this.conn.host}:${this.conn.port}` +
+      `/${encodeURIComponent(database)}?authSource=${encodeURIComponent(database)}&serverSelectionTimeoutMS=10000`;
+    let stdout: string;
+    try {
+      ({ stdout } = await run(
+        "mongosh",
+        [uri, "--quiet", "--eval",
+          "print(EJSON.stringify(db.runCommand({ connectionStatus: 1 }).authInfo.authenticatedUserRoles))"],
+        { timeout: 30_000 },
+      ));
+    } catch (err) {
+      const text = `${(err as { stderr?: string }).stderr ?? ""}${(err as { stdout?: string }).stdout ?? ""}`;
+      if (/Authentication failed|AuthenticationFailed/i.test(text)) return false;
+      throw err;
+    }
+    const roles = JSON.parse(stdout.trim()) as { role: string; db: string }[];
+    return roles.some((r) => r.role === "dbOwner" && r.db === database);
+  }
+
   /** Drop a database and its owning user, idempotently. Dropping the database evicts its data; the
    *  user is removed first so a re-grant of the same login starts clean. */
   async dropDatabaseAndUser(database: string, user: string): Promise<void> {