fix: point the nuget dependabot entry at the project directories

The nuget entry was set to "/", which has produced no PRs in the repo's ten
months. The root holds no .csproj, and the solution is .slnx, which Dependabot
does not read, so it discovered nothing there.

Point it at the three directories that do hold a .csproj instead. Central
Package Management means those resolve their versions from
Directory.Packages.props, which is the file the updates land in.

The pins had drifted meanwhile: TUnit 1.43.11 against 1.61.0 upstream,
Testcontainers.PostgreSql 4.11.0 against 4.13.0, Scalar.AspNetCore 2.14.0
against 2.16.15. These are minor bumps, which the patch-only ignore does not
cover, so they should have been offered.

Claude-Session: https://claude.ai/code/session_01Mzkzx3hJsXN26mZZ8RXdEX

Author: philippemnoel

.github/dependabot.yml

@@ -13,8 +13,14 @@ updates:
         patterns:
           - "*"
 
+  # The project files, not "/": the root has no .csproj and Dependabot does not read the .slnx
+  # solution format, so nothing was discovered there. Central Package Management means the versions
+  # these resolve to live in Directory.Packages.props, which is what actually gets updated.
   - package-ecosystem: "nuget"
-    directory: "/"
+    directories:
+      - "/src"
+      - "/tests"
+      - "/examples"
     schedule:
       interval: "monthly"
     ignore: