Document Unicode encoding behavior in Uri.GetLeftPart method

[Copilot]

The Uri.GetLeftPart method performs Unicode character encoding and normalization under the hood, which can cause subtle bugs when developers expect simple string manipulation. This was reported as an issue where users were using GetLeftPart to remove SAS tokens but discovered it does more than just string truncation.

Changes Made

Added comprehensive documentation to the Uri.GetLeftPart method to clarify:

1. Important warning: Added a prominent callout explaining that the method performs Unicode character encoding and normalization
2. Behavioral clarification: Explicitly states it's not a simple string manipulation method and that the returned string may differ from the original URI
3. Implementation details: Documents that GetLeftPart is equivalent to calling GetComponents with specific UriComponents flags
4. Example mapping: Shows how GetLeftPart(UriPartial.Authority) maps to the corresponding GetComponents call

Example of the documentation change:

> [!IMPORTANT]
> The GetLeftPart method performs Unicode character encoding and normalization as part of its processing. It is not a simple string manipulation method. The returned string may differ from the original URI string due to this encoding behavior.

The GetLeftPart method is equivalent to calling GetComponents with the appropriate UriComponents flags. For example:
- `GetLeftPart(UriPartial.Authority)` is equivalent to `GetComponents(UriComponents.Scheme | UriComponents.UserInfo | UriComponents.Host | UriComponents.Port, UriFormat.UriEscaped)`

This documentation will help developers understand the method's true behavior and avoid unexpected results when using it for scenarios like removing query parameters or fragments from URIs.

Fixes #11253.

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[jeffhandley]

Looks good to me. @dotnet/ncl, how about someone from your team?