Add a `pformat_caller_frame()` render guard test

`pformat_boxed_tb()` has never accepted an `indent` kwarg but
`pformat_caller_frame(box_tb=True)` has been passing one since
`888af602`. Nothing in the suite covered the branch, so the
`TypeError` only ever surfaced from `_mk_send_mte()` — i.e.
EVERY send-side `MsgTypeError` blew up while formatting itself
and masked the real msg-spec violation behind a bogus
`TypeError`.

Red on purpose per the test-first convention; the 1-line fix
lands next.

Also pin `pformat_boxed_tb()`s signature so a future typo'd
kwarg fails loudly at the call site instead of only when some
rare error path runs.

(this patch was generated in some part by `claude-code` using `claude-opus-5` (`anthropic`))

tests/devx/test_pformat.py

@@ -0,0 +1,52 @@
+'''
+Unit tests for the `tractor.devx.pformat` render helpers.
+
+'''
+from __future__ import annotations
+
+import pytest
+
+from tractor.devx.pformat import (
+    pformat_boxed_tb,
+    pformat_caller_frame,
+)
+
+
+@pytest.mark.parametrize(
+    'box_tb',
+    [True, False],
+    ids=['boxed', 'bare'],
+)
+def test_pformat_caller_frame_renders(box_tb: bool):
+    '''
+    `pformat_caller_frame()` must render, not raise.
+
+    XXX the `box_tb=True` branch was passing an `indent=''` kwarg
+    that `pformat_boxed_tb()` never accepted, so it blew up with
+    a `TypeError`. Nothing in the test suite covered it, and the
+    only caller is `_mk_send_mte()` — i.e. EVERY send-side
+    `MsgTypeError` died while formatting itself, masking the real
+    msg-spec violation behind a bogus `TypeError`.
+
+    '''
+    report: str = pformat_caller_frame(
+        stack_limit=3,
+        box_tb=box_tb,
+    )
+    assert isinstance(report, str)
+    assert 'test_pformat_caller_frame_renders' in report
+
+
+def test_pformat_boxed_tb_rejects_unknown_kwargs():
+    '''
+    Pin the signature so a future typo'd kwarg fails loudly at the
+    call site rather than only when some rare error path runs.
+
+    '''
+    assert pformat_boxed_tb(tb_str='doggy\n')
+
+    with pytest.raises(TypeError):
+        pformat_boxed_tb(
+            tb_str='doggy\n',
+            indent='',
+        )