Fix prompt() per-call kwargs permanently changing session state

[Fridayai700]

Summary

Fixes #967. When calling session.prompt(is_password=True), the is_password attribute was permanently set on the session, affecting all subsequent prompt() calls. The same issue applied to every other kwarg accepted by prompt() and prompt_async().

Changes

• Save original values of overridden attributes before applying per-call kwargs
• Restore them in a finally block after prompt() / prompt_async() returns (or raises)
• Remove duplicate is_password entry in the _fields tuple
• Add test verifying attributes are restored after a per-call override

Before

session = PromptSession()
session.prompt('Password: ', is_password=True)
# session.is_password is now True permanently
session.prompt('Name: ')  # still masked!

After

session = PromptSession()
session.prompt('Password: ', is_password=True)
# session.is_password is restored to False
session.prompt('Name: ')  # works as expected

Notes

The original code had a comment saying save/restore was removed because it was confusing and did not really serve a use case — but #967 shows it does. This implementation is minimal: it only saves/restores attributes that are actually passed as kwargs in a given call, so setting session.editing_mode = EditingMode.VI between calls still works as expected.