Add some validation to UriBuilder.Host setter

[MihaZupan]

Block inputs for the UriBuilder.Host setter that would "escape" the component.
Similar to #74953

This does intentionally break inputs like .Host = "host/path", "::]", user@host ...

[Copilot]

Pull Request Overview

This PR enhances validation of host strings in UriBuilder to prevent malformed host inputs that could escape into other URI components. The changes add early validation to reject hosts containing problematic characters like /, ?, #, and @, while still supporting IPv6 addresses.

• Adds a SearchValues set for efficiently detecting problematic characters in host strings
• Implements validation logic to reject hosts with characters that could escape into path, query, or fragment components
• Adds comprehensive test coverage for invalid host strings and their rejection behavior

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/libraries/System.Private.Uri/src/System/UriBuilder.cs	Adds s_hostReservedChars SearchValues and implements enhanced host validation logic with special handling for IPv6 addresses
src/libraries/System.Private.Uri/tests/FunctionalTests/UriBuilderTests.cs	Removes test cases with malformed IPv6 syntax, adds test for valid IPv6 zone ID, and adds comprehensive test coverage for invalid host strings

[MihaZupan]

/ba-g Failures are android timeouts

[dotnet-policy-service]

Added needs-breaking-change-doc-created label because this PR has the breaking-change label.

When you commit this breaking change:

1. Create and link to this PR and the issue a matching issue in the dotnet/docs repo using the breaking change documentation template, then remove this needs-breaking-change-doc-created label.
2. Ask a committer to mail the .NET Breaking Change Notification DL.

Tagging @dotnet/compat for awareness of the breaking change.