Refine idx adjustment: use newHost.Length directly

The correct adjustment when newHost replaces _string is simply idx = newHost.Length,
because newHost contains everything from the start through the end of the authority
with bidi chars removed, and idx should point to the position right after the authority.

This is more correct than the previous calculation (idx -= length - newHost.Length)
which incorrectly used the full URI length instead of just the authority portion.

Current test results:
- Percent-encoded userinfo tests: ✅ Pass (213/213)
- Bidi character tests: Improved but still issues (test framework shows "no exception"
  for some cases, but manual testing still shows IndexOutOfRangeException for others)

The issue appears to be that newHost.Length works for some cases but not others. Further
investigation needed into edge cases where the string building in CheckAuthorityHelper
doesn't produce the expected result.

Co-authored-by: MihaZupan <[email protected]>

Author: Copilot

src/libraries/System.Private.Uri/src/System/Uri.cs

@@ -2234,11 +2234,11 @@ private unsafe ParsingError PrivateParseMinimal()
 
                     if (newHost is not null)
                     {
-                        // When bidi characters are stripped, newHost is shorter than the original string.
-                        // Adjust idx to account for the removed characters.
-                        int removedChars = length - newHost.Length;
-                        idx -= removedChars;
                         _string = newHost;
+                        // newHost contains everything from the start through the end of the authority
+                        // with bidi characters removed. idx should point to the position right after
+                        // the authority in the new string, which is simply newHost.Length.
+                        idx = newHost.Length;
                     }
                 }