Improve HttpEnvironmentProxy uri parsing

[MihaZupan]

We have a test that the path may be present, but while that particular input happens to work, the parsing logic isn't actually accounting for paths as a possibility.
If the path/query/fragment contains : or @, it'd break.

This change strips out the path/query/fragment portion so that our host that we pass to UriBuilder is more likely to be just the host.
Hit in #121083 that tries to add more validation to the UriBuilder.Host setter.

This does break some weird edge cases that we happened to support currently, e.g. host:8080abc (garbage after the port).
But I kept other weird cases like user:p@ss@host as-is (@ as part of user info), which does mean that a @ in the path can still break things.

Ideally this method would be using Uri.TryCreate instead, but we currently accept some interesting inputs like domain\foo:pass@1.1.1.1, so we'd still have to do some manual processing.
dotnet/corefx#31232 (comment)

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[Copilot]

Pull Request Overview

This PR improves the parsing logic for HTTP proxy environment variables in HttpEnvironmentProxy. The changes fix issues with parsing proxy URLs that contain path, query, or fragment components, and simplify the logic for extracting user authentication information.

Key changes:

• Changed from LastIndexOf('@') to IndexOf('@') when parsing user authentication to avoid incorrect parsing when '@' appears in paths
• Added logic to strip path/query/fragment delimiters ('/', '?', '#') from proxy URLs before parsing
• Simplified port extraction by removing the manual digit-stripping loop and relying on TryParse to handle the remaining string
• Improved documentation clarity for the GetUriFromString method
• Optimized string comparison in bypass list matching
• Removed redundant using statements

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
HttpEnvironmentProxy.cs	Improved proxy URL parsing to handle path/query/fragment components, fixed user info extraction logic, simplified port parsing, and improved documentation
HttpEnvironmentProxyTest.cs	Added test cases to verify correct parsing of proxy URLs with user info containing '@', and URLs with path/query/fragment components

[MihaZupan]

/ba-g Android timeouts