test: replace ReflectorNet.dll with build from feature/atomic-api

The committed tests in this PR exercise the new atomic-modify and
view APIs (TryModifyAt, TryPatch, TryReadAt, View, Grep) introduced
by IvanMurzak/ReflectorNet#71 (HEAD 2252f623, branch feature/atomic-api).
Those APIs are not in the published com.IvanMurzak.ReflectorNet 5.0.0
NuGet, so building this PR against the released DLL fails compilation
on every Unity matrix job.

Replace the 5.0.0 DLL with a Release build of ReflectorNet at
feature/atomic-api SHA 2252f623 so CI compiles. The .csproj HintPath
locations are unchanged; only the binary content is updated.

This is a temporary cross-repo dependency stub. Once ReflectorNet#71
merges and a NuGet release ships with the atomic API surface, the
proper path is:

  1. Bump the Unity-MCP-Plugin NuGet folder name + .csproj HintPaths
     to the released ReflectorNet version (e.g. 5.1.0).
  2. Replace this dev DLL with the release artefact.

DLL provenance:
  - Source: ReflectorNet/ReflectorNet/bin/Release/netstandard2.1/ReflectorNet.dll
  - Built from: feature/atomic-api @ 2252f623495ff9355f022ca68990807b5280b429
  - md5: cdf5e92be0f2e5084d5b8ea16b57b4de
  - Size: 227328 bytes (was 206336 bytes for released 5.0.0)

Author: IvanMurzak