Report fatal explicit-home startup failures (#9227)

* Report fatal explicit-home encodings initialization failures

Retain explicit standard-library initialization errors for the command-line host, preserving default embedding fallback and the original exception cause.

Assisted-by: OpenAI Codex: model version unavailable

* Enable canonical fatal startup path configuration coverage

Retire the verified expected failure through update_lib, preserving the explicit local path-environment adaptation and all other markers.

Assisted-by: OpenAI Codex: model version unavailable

Lib/test/test_cmd_line.py

@@ -1369,7 +1369,6 @@ class CmdLineTest(unittest.TestCase):
         proc = assert_python_ok("-X", f"presite={entrypoint}", "-c", "pass")
         self.assertEqual(proc.out.rstrip(), b"presite func")
 
-    @unittest.expectedFailure  # TODO: RUSTPYTHON; explicit PYTHONHOME startup path configuration and fatal encodings failure reporting are not implemented
     def test_dump_path_config(self):
         # gh-151253: At the first import (import encodings) during Python
         # startup, if the import fails, dump the Python path configuration.