Remove most preemptive checks from GetFullPath (#16478)

Remove most preemptive checks from GetFullPath (#16478)

* Removing the colon block that tosses out paths that are not device path.

Fixes: #26359

* Striping 2nd and 3rd args of PathHelper.Normalize

* Build on Maryam's changes

- Remove all path validity checks outside of
    - Null
    - Embedded null
    - Empty or all spaces (effectively empty)
- Remove PathStartSkip helper
- Use span overloads for StringBuffer usage
- Clean up some comments

* Address feedback

* Tweak to match other AsSpan methods