Fix mismatched traceback in pythonrc.py injection (#25480)

Since vscode-python injects `PYTHONSTARTUP` into `pythonrc.py` to show
the "Ctrl click to launch VS Code Native REPL" message, traceback always
shows lines inside `pythonrc.py` in the interactive console.

Example:

<img width="1199" height="732" alt="image"
src="https://github.com/user-attachments/assets/7c68a20b-7b3c-4f32-bc76-3e73a61282e5"
/>

That's because python sets `__main__` to the `pythonrc.py` module, which
has a `__loader__` attribute. The `__loader__` object is then exposed in
the global namespace of the interactive console. However, [linecache
uses `__loader__` to get the source
code](https://github.com/python/cpython/blob/a756a4b95341cda63298fb5628b56c32f43f4f1e/Lib/linecache.py#L231),
causing the mismatch.

After this PR, the traceback points to the right source now:

<img width="1223" height="488" alt="image"
src="https://github.com/user-attachments/assets/b4d1635b-9137-463d-91ab-34b7d178e98d"
/>

In the [`_pyrepl.__main__`
module](https://github.com/python/cpython/blob/main/Lib/_pyrepl/__main__.py),
there was some comment mentioning this behavior and they set `__loader__
= None` to avoid this. See python/cpython#129098 for more information

---------

Co-authored-by: Eleanor Boyd <26030610+eleanorjboyd@users.noreply.github.com>
Co-authored-by: Eduardo Villalpando Mello <eduardovil@microsoft.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: 4 people

python_files/pythonrc.py

@@ -1,3 +1,10 @@
+# PYTHONSTARTUP runs in __main__; clear its metadata at module scope so linecache
+# cannot use this file's loader for later interactive tracebacks.
+# CPython issue: https://github.com/python/cpython/issues/129098
+# CPython fix: https://github.com/python/cpython/pull/130721
+__spec__ = __loader__ = None
+
+
 def _initialize():
     import platform
     import sys

python_files/tests/test_shell_integration.py

@@ -1,6 +1,8 @@
 import importlib
+import linecache
 import platform
 import sys
+from importlib.machinery import ModuleSpec, SourceFileLoader
 from pathlib import Path
 from typing import Any, Protocol, cast
 from unittest.mock import Mock
@@ -81,6 +83,23 @@ def test_does_not_pollute_namespace():
     assert not [name for name in vars(pythonrc) if not name.startswith("__")]
 
 
+def test_pythonstartup_clears_loader_metadata_for_tracebacks(monkeypatch, tmp_path):
+    source = PYTHONRC_PATH.read_text(encoding="utf-8")
+    loader = SourceFileLoader("__main__", str(PYTHONRC_PATH))
+    namespace: dict[str, Any] = {
+        "__name__": "__main__",
+        "__loader__": loader,
+        "__spec__": ModuleSpec("__main__", loader),
+    }
+    monkeypatch.setattr(linecache, "cache", {})
+
+    exec(compile(source, str(PYTHONRC_PATH), "exec"), namespace)
+
+    assert linecache.getline(str(tmp_path / "missing.py"), 1, namespace) == ""
+    assert namespace["__loader__"] is None
+    assert namespace["__spec__"] is None
+
+
 def test_replacement_regex_removes_bel_terminated_osc():
     if sys.version_info < (3, 13):
         return