Fix Uri.TryUnescapeDataString throwing instead of returning false with small destination buffers

[Copilot]

Uri.TryUnescapeDataString threw ArgumentOutOfRangeException instead of returning false when the destination span was shorter than the index of the first % in the input:

Uri.TryUnescapeDataString("aa%", [], out _); // threw ArgumentOutOfRangeException

Description

Root cause: When the input contains %, the method uses destination.Slice(indexOfFirstToUnescape) as the initial buffer for ValueStringBuilder. If destination.Length < indexOfFirstToUnescape, this Slice call throws instead of returning false gracefully.

Fix: Added an early-out guard — when destination.Length < indexOfFirstToUnescape, the destination cannot hold even the literal prefix before the first %, so false is returned immediately with charsWritten = 0. This check is placed before the overlapped-buffer detection, so it fires unconditionally in both overlapped and non-overlapped cases.

Tests: Added [Theory] cases covering the reported scenario and similar variants ("aa%" and "aaa%41" with destination sizes smaller than the prefix length).

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Uri.TryUnescapeDataString may throw with small destination buffers</issue_title>
<issue_description>```c#
Uri.TryUnescapeDataString("aa%", [], out _)

Throws `ArgumentOutOfRangeException` instead of returning `false`.</issue_description>

## Comments on the Issue (you are @copilot in this section)

<comments>
</comments>

• Fixes Uri.TryUnescapeDataString may throw with small destination buffers #124654

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[Copilot]

Pull request overview

This PR fixes a bug in Uri.TryUnescapeDataString where the method threw ArgumentOutOfRangeException instead of returning false when the destination buffer was smaller than the index of the first % character in the input. The fix adds an early-out guard that checks if the destination can hold the literal prefix before the first escape sequence.

Changes:

• Added an early-out check in TryUnescapeDataString to return false when destination.Length < indexOfFirstToUnescape
• Added test cases covering the bug scenario with various input strings and destination sizes

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/UriExt.cs	Added guard check to prevent ArgumentOutOfRangeException when destination is too small to hold the literal prefix before first %
src/libraries/System.Private.Uri/tests/FunctionalTests/UriEscapingTest.cs	Added [Theory] test with 5 test cases covering the bug scenario with different input strings and destination buffer sizes