Add `tests/msg/` sub-pkg, audit `pformat()` filtering

Reorganize existing msg-related test suites under
a new `tests/msg/` subdir (matching `tests/devx/`
and `tests/ipc/` convention) and add unit tests for
the `_`-prefixed field filtering in `pformat()`.

Deats,
- `git mv` `test_ext_types_msgspec` and `test_pldrx_limiting` into
  `tests/msg/`.
- add `__init__.py` + `conftest.py` for the new test sub-pkg.
- add new `test_pretty_struct.py` suite with 8 unit tests:
  - parametrized field visibility (public shown, `_`-private hidden,
    mixed)
  - direct `iter_struct_ppfmt_lines()` assertion
  - nested struct recursion filtering
  - empty struct edge case
  - real `MsgDec` via `mk_dec()` hiding `_dec`
  - `repr()` integration via `Struct.__repr__()`

(this patch was generated in some part by [`claude-code`][claude-code-gh])
[claude-code-gh]: https://github.com/anthropics/claude-code

tests/msg/__init__.py

@@ -0,0 +1,4 @@
+'''
+`tractor.msg.*` sub-sys test suite.
+
+'''

tests/msg/conftest.py

@@ -0,0 +1,4 @@
+'''
+`tractor.msg.*` test sub-pkg conf.
+
+'''

tests/msg/test_pretty_struct.py

@@ -0,0 +1,244 @@
+'''
+Unit tests for `tractor.msg.pretty_struct`
+private-field filtering in `pformat()`.
+
+'''
+from typing import (
+    Any,
+)
+
+import pytest
+
+from tractor.msg.pretty_struct import (
+    Struct,
+    pformat,
+    iter_struct_ppfmt_lines,
+)
+from tractor.msg._codec import (
+    MsgDec,
+    mk_dec,
+)
+
+
+# ------ test struct definitions ------ #
+
+class PublicOnly(Struct):
+    '''
+    All-public fields for baseline testing.
+
+    '''
+    name: str = 'alice'
+    age: int = 30
+
+
+class PrivateOnly(Struct):
+    '''
+    Only underscore-prefixed (private) fields.
+
+    '''
+    _secret: str = 'hidden'
+    _internal: int = 99
+
+
+class MixedFields(Struct):
+    '''
+    Mix of public and private fields.
+
+    '''
+    name: str = 'bob'
+    _hidden: int = 42
+    value: float = 3.14
+    _meta: str = 'internal'
+
+
+class Inner(
+    Struct,
+    frozen=True,
+):
+    '''
+    Frozen inner struct with a private field,
+    for nesting tests.
+
+    '''
+    x: int = 1
+    _secret: str = 'nope'
+
+
+class Outer(Struct):
+    '''
+    Outer struct nesting an `Inner`.
+
+    '''
+    label: str = 'outer'
+    inner: Inner = Inner()
+
+
+class EmptyStruct(Struct):
+    '''
+    Struct with zero fields.
+
+    '''
+    pass
+
+
+# ------ tests ------ #
+
+@pytest.mark.parametrize(
+    'struct_and_expected',
+    [
+        (
+            PublicOnly(),
+            {
+                'shown': ['name', 'age'],
+                'hidden': [],
+            },
+        ),
+        (
+            MixedFields(),
+            {
+                'shown': ['name', 'value'],
+                'hidden': ['_hidden', '_meta'],
+            },
+        ),
+        (
+            PrivateOnly(),
+            {
+                'shown': [],
+                'hidden': ['_secret', '_internal'],
+            },
+        ),
+    ],
+    ids=[
+        'all-public',
+        'mixed-pub-priv',
+        'all-private',
+    ],
+)
+def test_field_visibility_in_pformat(
+    struct_and_expected: tuple[
+        Struct,
+        dict[str, list[str]],
+    ],
+):
+    '''
+    Verify `pformat()` shows public fields
+    and hides `_`-prefixed private fields.
+
+    '''
+    (
+        struct,
+        expected,
+    ) = struct_and_expected
+    output: str = pformat(struct)
+
+    for field_name in expected['shown']:
+        assert field_name in output, (
+            f'{field_name!r} should appear in:\n'
+            f'{output}'
+        )
+
+    for field_name in expected['hidden']:
+        assert field_name not in output, (
+            f'{field_name!r} should NOT appear in:\n'
+            f'{output}'
+        )
+
+
+def test_iter_ppfmt_lines_skips_private():
+    '''
+    Directly verify `iter_struct_ppfmt_lines()`
+    never yields tuples with `_`-prefixed field
+    names.
+
+    '''
+    struct = MixedFields()
+    lines: list[tuple[str, str]] = list(
+        iter_struct_ppfmt_lines(
+            struct,
+            field_indent=2,
+        )
+    )
+    # should have lines for public fields only
+    assert len(lines) == 2
+
+    for _prefix, line_content in lines:
+        field_name: str = (
+            line_content.split(':')[0].strip()
+        )
+        assert not field_name.startswith('_'), (
+            f'private field leaked: {field_name!r}'
+        )
+
+
+def test_nested_struct_filters_inner_private():
+    '''
+    Verify that nested struct's private fields
+    are also filtered out during recursion.
+
+    '''
+    outer = Outer()
+    output: str = pformat(outer)
+
+    # outer's public field
+    assert 'label' in output
+
+    # inner's public field (recursed into)
+    assert 'x' in output
+
+    # inner's private field must be hidden
+    assert '_secret' not in output
+
+
+def test_empty_struct_pformat():
+    '''
+    An empty struct should produce a valid
+    `pformat()` result with no field lines.
+
+    '''
+    output: str = pformat(EmptyStruct())
+    assert 'EmptyStruct(' in output
+    assert output.rstrip().endswith(')')
+
+    # no field lines => only struct header+footer
+    lines: list[tuple[str, str]] = list(
+        iter_struct_ppfmt_lines(
+            EmptyStruct(),
+            field_indent=2,
+        )
+    )
+    assert lines == []
+
+
+def test_real_msgdec_pformat_hides_private():
+    '''
+    Verify `pformat()` on a real `MsgDec`
+    hides the `_dec` internal field.
+
+    NOTE: `MsgDec.__repr__` is custom and does
+    NOT call `pformat()`, so we call it directly.
+
+    '''
+    dec: MsgDec = mk_dec(spec=int)
+    output: str = pformat(dec)
+
+    # the private `_dec` field should be filtered
+    assert '_dec' not in output
+
+    # but the struct type name should be present
+    assert 'MsgDec(' in output
+
+
+def test_pformat_repr_integration():
+    '''
+    Verify that `Struct.__repr__()` (which calls
+    `pformat()`) also hides private fields for
+    custom structs that do NOT override `__repr__`.
+
+    '''
+    mixed = MixedFields()
+    output: str = repr(mixed)
+
+    assert 'name' in output
+    assert 'value' in output
+    assert '_hidden' not in output
+    assert '_meta' not in output