Fix #26039: REPL prompt disappears when builtins/globals shadowed under PYTHONSTARTUP

[mohityadav8]

Fixes #26039

PYTHONSTARTUP executes pythonrc.py's code directly inside the user's __main__ namespace rather than importing it as a module. That means PS1.__str__.__globals__ is the user's namespace — so shadowing any name it relies on at prompt-render time (int, sys, str, bool, original_ps1, get_last_command) breaks str(sys.ps1) and silently kills the prompt.

This captures the real objects into private _-prefixed aliases right after they're defined, before any user code runs, so later reassignment of those names in __main__ can't affect the prompt anymore.

How I tested:

• Reproduced by exec-ing pythonrc.py's source into a synthetic __main__ dict (mirroring the real PYTHONSTARTUP path) and shadowing int/sys/str/bool/original_ps1/get_last_command — confirmed it broke on the original file and is fixed on the patched one.
• Added test_prompt_survives_shadowed_builtins_under_pythonstartup, which encodes that reproduction as a regression test. The existing tests import pythonrc as a normal module, which gives PS1 its own module namespace instead of __main__ — that's why they never caught this bug.
• npm run check-python (ruff check, ruff format --check, pyright) — clean, 0 errors.
• python -m pytest python_files/tests/test_shell_integration.py -v — 5/5 passed.

Notes for reviewers: the fix is intentionally minimal (capture at definition time) rather than moving the class into a separate module, since pythonStartup.ts only ever copies the single pythonrc.py file to the PYTHONSTARTUP location — splitting into two files would require extension-side deployment changes.