Add unit tests for MarshalerInputStream

[Vicky677]

Description

Adds comprehensive unit test coverage for io.opentelemetry.exporter.internal.grpc.MarshalerInputStream in opentelemetry-exporter-common.

Test scenarios covered:

• available():
  • Checking available bytes when backed by a MessageWriter.
  • Checking available bytes when backed by a partial ByteArrayInputStream.
  • Returning 0 after the stream has been fully drained.
• drainTo(OutputStream):
  • Draining directly from a MessageWriter.
  • Draining from an already existing partial stream.
  • Returning 0 written bytes when called on an already drained stream.
• read() (single byte):
  • Reading single bytes sequentially until EOF (-1) from a MessageWriter.
  • Reading single bytes from a partial stream.
  • Returning -1 on already drained stream.
• read(byte[], int, int) (bulk read):
  • Reading bulk byte array from a MessageWriter and verifying EOF on subsequent read.
  • Handling 0-length messages properly.
  • Reading bulk bytes from a partial stream.
  • Returning -1 on already drained stream.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: Vicky677 / name: Abhishek.Ghosh11 (38251b8)