Fix empty host parsing regression

[MihaZupan]

Fixes #124426
Regressed in #121671

The previous check had end < length, which wasn't so much a range check as a check that end was updated

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

Line 3967 in 7fb0240

else if (end < length && pString[end] == ':')

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[Copilot]

Pull request overview

This PR fixes a regression introduced in #121671 where URIs with no host but with a port (e.g., tcp://:11111) were incorrectly accepted instead of throwing a UriFormatException. The fix restores the original behavior by adding a check to ensure that port parsing only occurs when a non-empty host is present.

Changes:

• Fixed the host parsing logic to require a non-zero hostLength before attempting to parse a port
• Added comprehensive test coverage for unknown schemes with empty hosts followed by ports

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
src/libraries/System.Private.Uri/src/System/Uri.cs	Added && hostLength != 0 condition to port parsing check to prevent parsing ports when no host is present
src/libraries/System.Private.Uri/tests/FunctionalTests/UriCreateStringTests.cs	Added test cases for unknown schemes with empty hosts and ports, including variations with paths, queries, and fragments

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.