gh-157006: Clear PYTHON* env vars in test_embed

[ghost]

My test_embed was failing because VS Code's shell integration sets
PYTHONSTARTUP in the environment. The embedded interpreter picks it up
and runs the startup script, which prints extra text before the
expected "ok! Py_RunMain() returned 123", causing the assertion to
fail.

Detailed root cause analysis:

VS Code shell integration sets PYTHONSTARTUP to a script that prints
something like "Ctrl click to launch VS Code Native REPL". When
test_embed runs _testembed via run_embedded_interpreter(), the
method passes env=None to subprocess.Popen, which inherits the full
parent environment. The embedded interpreter then sees PYTHONSTARTUP
and executes the startup script before running the actual test code.
The startup script's output is prepended to stdout, so
check_program_exitcode() receives:

"Ctrl click to launch VS Code Native REPL\nok! Py_RunMain() returned 123"

instead of the expected:

"ok! Py_RunMain() returned 123"

The assertion self.assertEqual(out.rstrip(), 'ok! Py_RunMain() returned 123')
then fails.

This only affects tests that go through run_embedded_interpreter()
without passing an explicit env. Tests that already call
remove_python_envvars() (e.g., the config snapshot tests around
line 878) are not affected.

There is also a secondary issue in test_init_run_main_startup_exitcode:
it builds its env from dict(os.environ), which copies the parent's
PYTHONSTARTUP before overriding it with the test's own startup file.
If the parent's PYTHONSTARTUP script produces output, it would also
pollute the test's expected output.

Changes:

• run_embedded_interpreter(): when env is None, default to
  remove_python_envvars() instead of inheriting os.environ. This
  strips all PYTHON* variables (PYTHONSTARTUP, PYTHONPATH, PYTHONHOME,
  etc.) that could affect the embedded interpreter's behavior. This is
  consistent with how other tests in the same file already isolate
  themselves from the parent environment.

• test_init_run_main_startup_exitcode: changed dict(os.environ) to
  remove_python_envvars() as the base env, then sets PYTHONSTARTUP
  to the test's own startup file. This ensures the test is fully
  isolated from whatever PYTHON* vars the runner happens to have.

Closes #157006

Note: this is a resubmission of #157008. The description on that PR
was a rough draft that I hadn't finished polishing when it was closed.

Note: AI tools were used only to assist in problem analysis. The fix
and code changes were determined and verified independently.

• Issue: test_embed may fail if PYTHONSTARTUP set #157006