BaseEncoding: Make encodingStream().close() idempotent

[nickita-khylkouski]

Summary

• Add boolean closed field to the OutputStream returned by encodingStream()
• Make write(), flush(), and close() synchronized per @cpovirk's guidance
• close() returns early if already closed (idempotent)
• write()/flush() throw IOException after close, matching AppendableWriter and CharSequenceReader patterns

Motivation

BaseEncoding.encodingStream().close() violates the Closeable.close() contract which states: "If the stream is already closed then invoking this method has no effect."

Calling close() twice writes additional encoded data to the underlying Writer:

StringWriter w = new StringWriter();
OutputStream out = BaseEncoding.base64().encodingStream(w);
out.write(0);
out.close();
System.out.println(w); // "AA==" (correct)
out.close();
System.out.println(w); // "AA==A===" (wrong - extra data)

This is problematic because double-close is common due to try-with-resources blocks and wrapping streams that close their underlying streams.

Fixes #5284

Testing

• 3 new tests in BaseEncodingTest:
  • testEncodingStreamCloseIsIdempotent — verifies close() twice doesn't change output
  • testEncodingStreamWriteAfterClose — verifies write() throws IOException after close
  • testEncodingStreamFlushAfterClose — verifies flush() throws IOException after close
• All 43 tests in BaseEncodingTest pass (./mvnw test -pl guava-tests -Dtest=BaseEncodingTest)
• Added IOException to ReflectionFreeAssertThrows to support the new test assertions