Improve ASGI script prefix path_info handling

[Khadyot Takale]

The current ASGIRequest.init uses str.removeprefix() to strip the script name from the request path to compute path_info. This is fragile because removeprefix is a pure string operation — it doesn't verify that the prefix is a proper path segment boundary.

For example, if script_name is /myapp and the path is /myapplication/page, removeprefix would incorrectly produce lication/page.

This patch replaces removeprefix with a check that ensures the script name is followed by / or is the exact path, before stripping it. This addresses the inline TODO comment.

[Jacob Walls <jacobtylerwalls@…>]

In b33c31d:

Fixed #36940 -- Fixed script name edge case in ASGIRequest.path_info.

Paths that happened to begin with the script name were inappropriately
stripped, instead of checking that script name preceded a slash.