GH-939: Remove reflection for gRPC buffers

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What's Changed

• The branch removes the previous reflection-based access to internal gRPC buffers and replaces it with supported gRPC interfaces (Detachable, HasByteBuffer) to safely transfer ownership of direct buffers to Arrow without copying.
• gRPC direct ByteBuffer streams are detached and wrapped as ForeignAllocation for zero‑copy reads
• Fall back to safe buffer copies when gRPC does not expose a direct ByteBuffer or data is fragmented
• Added test coverage

Closes #939.