Show stdin traceback lines; omit StopIteration None args (#8778)

* Show stdin traceback lines; omit StopIteration None args

Register interactive <stdin> source in linecache so REPL
tracebacks include the source line and caret. Build
StopIteration with empty args when the value is None, and
wrap monitoring STOP_ITERATION the same way.

Unskip REPL stderr flush tests and pdb Flaky doctests.

Assisted-by: Grok:4.6

* Take &Py<PyCode> in register_code_in_linecache

Assisted-by: Grok:4.6

* Count Unknown closing braces in tokenize EOF check

`$` inside an f-string interpolation is TokenKind::Unknown, so
the matching `}` is Unknown rather than Rbrace and generate_tokens
raised TokenError. Also convert &Py<PyCode> to PyObjectRef
explicitly when registering linecache.

Assisted-by: Grok:4.6

* Unskip test_pdb.PdbTestCase.test_zipimport

The test now passes, so the expectedFailure is an unexpected
success under CI.

Assisted-by: Grok:4.6

* Park STW threads; poll sockets after timeout deadline

Check per-thread stop_requested in eval_breaker_tripped so
workers still park when another interpreter clears STOP_BIT.

Start socket timeout after the fd snapshot, and poll once with
timeout 0 when the deadline has already passed, so a readable
socket is not dropped as timed out.

Assisted-by: Grok:4.6

Lib/test/test_cmd_line_script.py

@@ -214,11 +214,9 @@ class CmdLineTest(unittest.TestCase):
     def test_repl_stdout_flush_separate_stderr(self):
         self.check_repl_stdout_flush(True)
 
-    @unittest.skip("TODO: RUSTPYTHON; <stdin> traceback omits the source line and caret")
     def test_repl_stderr_flush(self):
         self.check_repl_stderr_flush()
 
-    @unittest.skip("TODO: RUSTPYTHON; <stdin> traceback omits the source line and caret")
     def test_repl_stderr_flush_separate_stderr(self):
         self.check_repl_stderr_flush(True)