Make BaseEncoding.encodingStream().close() idempotent.

[Divyansh151005]

Problem

Closeable.close() specifies that closing an already-closed stream must have no effect. Today, calling close() twice on a BaseEncoding.encodingStream() can write extra padding characters on the second call.

Reproduction:

StringWriter w = new StringWriter();
OutputStream out = BaseEncoding.base64().encodingStream(w);
out.write(0);
out.close();
System.out.println(w); // AA==
out.close();
System.out.println(w); // AA==A=== (incorrect)

Fixes #5284

Solution

Track a closed flag on the encoding stream and return immediately from subsequent close() calls. Reset bitBufferLength after flushing any remaining bits on the first close.

Testing

$ export JAVA_HOME=~/.m2/jdks/jdk-26.0.2+10/Contents/Home
$ ./mvnw -B -Dtoolchain.skip -P!standard-with-extra-repos test \
    -Dmaven.javadoc.skip=true -Dsurefire.toolchain.version=17 \
    -Dtest=BaseEncodingTest#testEncodingStream_closeIsIdempotent \
    -DfailIfNoTests=false -pl guava-tests -am

[INFO] Running com.google.common.io.BaseEncodingTest
[INFO] Tests run: 1, Failures: 0, Errors: 0, Skipped: 0
[INFO] BUILD SUCCESS

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.