"[Errno 2] No such file or directory: b'net'" when trying to use Ansible localhost to connect to a Windows Shared Folder

The error messages says that the binary net is not found on the system. Hence, b'net'.
This is an expected output because the net command is not available on Linux, it is a Windows command.

How to mount a Windows share in Linux manually you can find in this article:
How to mount Windows share on Red Hat Enterprise Linux system using CIFS?

Since you want to do it in Ansible on the localhost by judging your output, you can do something like this:

mount_samba_localhost.yml:

- name: Setup SMB share
  hosts: localhost
  become: yes
  vars:
    username: smb_username
    password: smb_password
    domain: domain
    share_path: '//10.15.250.110/mixeddocs'
    mount_point: /mnt/my_mount_point
    credentials_file: /root/smb_credentials

  tasks:
    - name: Install cifs-utils
      ansible.builtin.package:
        name: cifs-utils
        state: present

    - name: Create directory for mount point
      ansible.builtin.file:
        path: "{{ mount_point }}"
        state: directory

    - name: Create credentials file
      ansible.builtin.copy:
        dest: "{{ credentials_file }}"
        content: |
          username={{ username }}
          password={{ password }}
          domain={{ domain }}
        mode: 0600
        owner: root
        group: root

    - name: Mount Windows share folder
      ansible.posix.mount:
        path: "{{ mount_point }}"
        src: "{{ share_path }}"
        fstype: cifs
        opts: 'credentials={{ credentials_file }}'
        state: mounted

This creates a credentials file in the root directory. In general, it is a best practice to avoid adding plain text passwords in the fstab file for security reasons. That's why a separate credentials file is created. Furthermore, an entry will be added to the fstab to ensure the mount persists across system reboots by ansible.posix.mount.

Though, I recommend using NFS-shares instead of CIFS.

Have fun!