Ensure built appose environments use newest appose

Normally, when building an environment in-line with uv -- i.e. with
include rather than a separate environment file -- the most recent
*release* version of appose will be used.

But in case there have been changes to the appose.python_worker here,
we'd like to be running our tests against this same *dev* version of
appose, rather than the older release version.

One way of encouraging that without making the appose builder logic
more complex is to inject the local appose source tree into the
built environment via a PYTHONPATH environment variable override.
Then this source tree will take precedence over the installed appose.

This commit adds a new source_override() helper function to test_base
so that tests needing it can do this with a single function call.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: ctrueden

tests/test_base.py

@@ -7,6 +7,7 @@
 from __future__ import annotations
 
 import os
+from pathlib import Path
 
 from appose import Environment
 from appose.service import ResponseType, Task, TaskStatus
@@ -123,6 +124,25 @@ def cowsay_and_assert(env: Environment, greeting: str):
         assert "||----w |" in actual, "Output should contain cow legs"
 
 
+def source_override() -> dict[str, str]:
+    """
+    Return a PYTHONPATH env var override pointing to the local appose source tree.
+
+    When appose is installed from source (editable install), worker processes in
+    built environments would otherwise use their own PyPI-installed copy of appose.
+    Passing this dict to builder.env() ensures workers load the same python_worker.py
+    that the test runner is using.
+
+    Returns an empty dict when appose is installed from PyPI (non-editable), so it
+    is safe to call unconditionally.
+    """
+    import appose
+
+    if "site-packages" not in Path(appose.__file__).parts:
+        return {"PYTHONPATH": str(Path(appose.__file__).parent.parent)}
+    return {}
+
+
 def maybe_debug(service):
     """
     Enable debug output if DEBUG or appose.debug is set.