Fix edge-case Uri debug assert failure

[MihaZupan]

If Compress removes the entire path, and that path was otherwise missing a leading slash, we can get into the case where dest.Length == start after the Compress call.
This triggers a debug assert in the ValueStringBuilder's this[] accessor since we're reading past the length, but it's actually harmless in practice in this case since the underlying buffer will always have at least 1 available character after start on this path (note the dest.Length - offset > 0 condition before compression).

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[wfurt]

nice.

[Copilot]

Pull request overview

This PR fixes a debug assertion failure in Uri path canonicalization that occurs when the Compress method removes an entire path that was missing a leading slash. The fix adds a bounds check before accessing the character at the start position.

• Adds bounds check (dest.Length > start) before accessing dest[start] to prevent reading past buffer length
• Adds braces for consistency with coding style
• Includes a test case that reproduces the edge case scenario

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	Adds bounds check and braces to prevent out-of-bounds access when path compression removes entire path
src/libraries/System.Private.Uri/tests/FunctionalTests/UriRelativeResolutionTest.cs	Adds test case for FTP URI where entire path can be removed by compression