Core: Align ByteBufferInputStream read behavior

[fallintoplace]

What changes are included?

• Keep destination ByteBuffers in write mode after SingleBufferInputStream.read(ByteBuffer).
• Return -1 at EOF while preserving 0 for destinations without remaining space.
• Add shared coverage that verifies the same behavior for single-buffer and multi-buffer streams.

Why is this change necessary?

ByteBufferInputStream.wrap selects its implementation based on the number of source buffers. The single-buffer implementation flipped the destination after writing, while the multi-buffer implementation advanced its position normally. As a result, the destination state and EOF result depended on how the source was internally split.

Testing

• ./gradlew :iceberg-core:test --tests org.apache.iceberg.io.TestSingleBufferInputStream --tests org.apache.iceberg.io.TestMultiBufferInputStream
• ./gradlew :iceberg-core:spotlessCheck :iceberg-core:checkstyleMain :iceberg-core:checkstyleTest

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AI Disclosure

• Model: OpenAI language model
• Platform/Tool: OpenAI desktop app
• Human Oversight: fully reviewed
• Prompt Summary: Investigated inconsistent ByteBufferInputStream behavior, implemented a focused fix, and added regression coverage.