ci: install Pillow in nuget-publish; deflake FluentApi save test

Two unrelated CI bugs that surfaced after the recent merges:

1) NuGet Publish workflow failed at the "Optimize package icon" step with
   `ModuleNotFoundError: No module named 'PIL'`. The Python heredoc that
   trims + centres + resizes the icon imports Pillow but the step never
   installs it on the ubuntu-latest runner. Add a one-line `pip install`
   before the heredoc.

2) DocumentTests.FluentApi_ChainsMultipleOperations races on a shared
   output path. The test calls SaveAsync("fluent-test"), which resolves
   to <repoRoot>/output/fluent-test.pdf via FileOperations.GetOutputPath.
   In CI we run the test suite against three TFMs in parallel (net8.0,
   net9.0, net10.0), so three test-host processes try to create the same
   file simultaneously and one loses with
   `IOException: The process cannot access the file ... because it is
    being used by another process`. Switch to a Guid-suffixed filename so
   the three TFM hosts each write to their own file.

Both fixes are minimal: no production code changes, no test-suite-wide
refactor. Other tests with similar deterministic filenames can be fixed
the same way if they start flaking.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: ANcpLua

.github/workflows/nuget-publish.yml

@@ -24,6 +24,7 @@ jobs:
 
       - name: Optimize package icon
         run: |
+          python3 -m pip install --quiet --disable-pip-version-check Pillow
           python3 << 'EOF'
           from PIL import Image
           import os

CreatePdf.NET.Tests/DocumentTests.cs

@@ -162,14 +162,18 @@ public async Task SaveAsync_WithFilename_CreatesFile()
     [Fact]
     public async Task FluentApi_ChainsMultipleOperations()
     {
+        // Unique filename so parallel test-host processes (one per target framework)
+        // do not race on the same output file.
+        var filename = $"fluent-test-{Guid.NewGuid():N}";
+
         Func<Task> act = () => Pdf.Create()
             .AddText("Title", Dye.Black, TextSize.Large, TextAlignment.Center)
             .AddLine()
             .AddText("Body text")
             .AddLines(2)
             .AddPixelText("Pixel text", Dye.Red)
             .AddLine()
-            .SaveAsync("fluent-test");
+            .SaveAsync(filename);
 
         await act.Should().NotThrowAsync().ConfigureAwait(true);
     }