Fix broken external links in clients, vectors/s3, and dotnet SDK recipes

[claudespice]

Found by HTTP-checking every URL in every cookbook README (460 unique URLs; 297 non-Spice). Four returned a genuine 404. Grouped into one PR because they are one issue class — an external target that moved — and each fix is a single line.

Recipe	Link	Status
clients/java, clients/scala	arrow.apache.org/docs/java/reference/.../FlightSqlClient.html	404
vectors/s3	aws.amazon.com/s3/vectors/	404
client-sdk/spice-dotnet-sdk-sample	nuget.org/packages/SpiceAI.Client	404

Apache Arrow — clients/java, clients/scala

Arrow retired the /docs/java/ tree; Java docs now live at /java/. The old link also pointed at the FlightSqlClient javadoc while the link text reads "Apache Arrow Flight SQL JDBC" — and both recipes actually depend on the JDBC driver, not the Java client:

• clients/java/pom.xml → org.apache.arrow:flight-sql-jdbc-driver
• clients/scala/build.sbt → "org.apache.arrow" % "flight-sql-jdbc-driver" % "19.0.0"

Repointed both to https://arrow.apache.org/java/current/flight_sql_jdbc_driver.html, which matches the link text and the dependency.

Amazon S3 Vectors — vectors/s3

AWS moved the page under /s3/features/. Repointed to https://aws.amazon.com/s3/features/vectors/. (The other S3 Vectors links in this recipe point at spiceai.org/docs/components/vectors/s3_vectors and already resolve.)

NuGet package — client-sdk/spice-dotnet-sdk-sample

The README links SpiceAI.Client, which does not exist on NuGet. The sample's own project file already references the real package:

<!-- spice-dotnet-sdk-sample.csproj:12 -->
<PackageReference Include="SpiceAI" Version="0.3.0" />

$ curl -s 'https://azuresearch-usnc.nuget.org/query?q=spiceai' | jq -r '.data[].id'
SpiceAI

Repointed to https://www.nuget.org/packages/SpiceAI.

Verification

$ for u in https://arrow.apache.org/java/current/flight_sql_jdbc_driver.html \
           https://aws.amazon.com/s3/features/vectors/ \
           https://www.nuget.org/packages/SpiceAI; do
    echo "$(curl -s -o /dev/null -w '%{http_code}' -L "$u") $u"
  done
200 https://arrow.apache.org/java/current/flight_sql_jdbc_driver.html
200 https://aws.amazon.com/s3/features/vectors/
200 https://www.nuget.org/packages/SpiceAI

Remaining non-200s in the sweep were all checked and are not defects: bot-blocked hosts returning 403 (dev.mysql.com, platform.openai.com, x.ai, npmjs.com, timeanddate.com), and intentional placeholders or non-browsable endpoints (api.company.com, your-org.atlassian.net, github.com/user/repo/..., api.tvmaze.com/premium, helm.ngc.nvidia.com/nvidia, the huggingface.co/.../resolve/main BASE_URL variable, and URLs embedded in sample JSON output).