Make Uri non-ASCII hex unescaping a bit faster

[MihaZupan]

Avoid some bounds checks and use a table lookup instead of manual hex calculations.
Closes #121413, followup to #121016

public class UriTest
{
    private char[] _buffer = new char[1024];

    [Benchmark]
    [Arguments("%E4%BD%A0%E5%A5%BD")]
    [Arguments("%E4%BD%A0%E5%A5%BD%E4%BD%A0%E5%A5%BD%E4%BD%A0%E5%A5%BD%E4%BD%A0%E5%A5%BD")]
    public bool TryUnescapeDataString(string input) => Uri.TryUnescapeDataString(input, _buffer, out _);
}

Method	Toolchain	input	Mean	Error	Ratio
TryUnescapeDataString	\main\corerun.exe	%E4%BD%A0%E5%A5%BD	43.34 ns	0.454 ns	1.00
TryUnescapeDataString	\pr\corerun.exe	%E4%BD%A0%E5%A5%BD	40.43 ns	0.364 ns	0.93
TryUnescapeDataString	\main\corerun.exe	%E4%B(...)A5%BD [72]	103.86 ns	0.151 ns	1.00
TryUnescapeDataString	\pr\corerun.exe	%E4%B(...)A5%BD [72]	97.55 ns	0.203 ns	0.94

[azure-pipelines]

Azure Pipelines:
Successfully started running 3 pipeline(s).
13 pipeline(s) were filtered out due to trigger conditions.
There may be pipelines that require an authorized user to comment /azp run to run.

[dotnet-policy-service]

Tagging subscribers to this area: @karelz, @dotnet/ncl
See info in area-owners.md if you want to be subscribed.

[Copilot]

Pull request overview

This PR updates System.Private.Uri’s percent-unescape helper to reduce overhead when decoding non-ASCII UTF-8 bytes from %xx sequences, primarily by switching to HexConverter table lookups and tightening bounds checks in the hot loop.

Changes:

• Replaces manual hex-nibble decoding with HexConverter.FromChar lookups for %xx parsing.
• Adjusts the bounds check around reading % + 2 hex chars to a form intended to help the JIT elide bounds checks.
• Adds a guard intended to prevent invalid hex from slipping past the ASCII-range check (but the current guard is insufficient and introduces a correctness bug; see review comment).

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

[EgorBo]

is that a jit issue?