common: make PersistedProgress._lock a private attribute

[sigmaris]

About this change - What it does

Makes the _lock member of PersistedProgress into a Pydantic private attribute. This means it'll be excluded from (de)serialization.
This fixes the following problem on Pydantic 2.10.6, Python 3.12.3:

UploadEventProgressTracker Thread-11 DEBUG: Tracking upload event for file default/xlog/000000020000002E0000009D
TransferAgent Thread-9 ERROR: Problem in moving file: PosixPath('/var/lib/pghoard/default/xlog/000000020000002E000000A6'), need to retry
Traceback (most recent call last):
  File "/var/lib/pghoard/venv/lib/python3.12/site-packages/pghoard/transfer.py", line 481, in handle_upload
    storage.store_file_object(
  File "/var/lib/pghoard/venv/lib/python3.12/site-packages/rohmu/object_storage/sftp.py", line 235, in store_file_object
    bytes_written = self._put_object(
                    ^^^^^^^^^^^^^^^^^
  File "/var/lib/pghoard/venv/lib/python3.12/site-packages/rohmu/object_storage/sftp.py", line 262, in _put_object
    self.client.putfo(fl=fd, remotepath=target_path, callback=wrapper_upload_progress_fn)
  File "/var/lib/pghoard/venv/lib/python3.12/site-packages/paramiko/sftp_client.py", line 716, in putfo
    size = self._transfer_with_callback(
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/var/lib/pghoard/venv/lib/python3.12/site-packages/paramiko/sftp_client.py", line 684, in _transfer_with_callback
    callback(size, file_size)
  File "/var/lib/pghoard/venv/lib/python3.12/site-packages/rohmu/object_storage/sftp.py", line 259, in wrapper_upload_progress_fn
    upload_progress_fn(bytes_written, total_bytes)
  File "/var/lib/pghoard/venv/lib/python3.12/site-packages/rohmu/object_storage/base.py", line 168, in wrapper
    cb(progress - last_progress)
  File "/var/lib/pghoard/venv/lib/python3.12/site-packages/pghoard/transfer.py", line 485, in <lambda>
    upload_progress_fn=lambda n_bytes: upload_progress_fn(total_bytes_uploaded=n_bytes),
                                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/var/lib/pghoard/venv/lib/python3.12/site-packages/pghoard/transfer.py", line 149, in increment
    persisted_progress = PersistedProgress.read(metrics=self.metrics)
                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/var/lib/pghoard/venv/lib/python3.12/site-packages/pghoard/common.py", line 154, in read
    return cls()
           ^^^^^
  File "/var/lib/pghoard/venv/lib/python3.12/site-packages/pydantic/main.py", line 214, in __init__
    validated_self = self.__pydantic_validator__.validate_python(data, self_instance=self)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/var/lib/pghoard/venv/lib/python3.12/site-packages/pydantic/_internal/_model_construction.py", line 339, in init_private_attributes
    default = private_attr.get_default()
              ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/var/lib/pghoard/venv/lib/python3.12/site-packages/pydantic/fields.py", line 1166, in get_default
    return _utils.smart_deepcopy(self.default) if self.default_factory is None else self.default_factory()
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/var/lib/pghoard/venv/lib/python3.12/site-packages/pydantic/_internal/_utils.py", line 345, in smart_deepcopy
    return deepcopy(obj)  # slowest way when we actually might need a deepcopy
           ^^^^^^^^^^^^^
  File "/usr/lib/python3.12/copy.py", line 151, in deepcopy
    rv = reductor(4)
         ^^^^^^^^^^^
TypeError: cannot pickle '_thread.lock' object

Why this way

This seems like the correct way to specify an attribute that isn't a field, should be excluded from serialization, and initialised by a default_factory according to https://docs.pydantic.dev/latest/concepts/models/#private-model-attributes