fix(util-stream): warn when buffered readable encounters mixed Buffer/Uint8Array chunks

[TrevorBurnham]

Issue #, if available:

Resolves #1553

Description of changes:

When a stream mixes Uint8Array and Buffer chunk types, createBufferedReadable flushes its internal buffer on every type switch regardless of size. This can produce undersized chunks that downstream consumers (like S3's chunked upload) reject with InvalidChunkSizeError, which is confusing when the user has explicitly set requestStreamBufferSize.

This change adds a logger.warn() when the buffering logic detects a switch between Buffer (mode 2) and Uint8Array (mode 1) chunks, in both the Node Readable and ReadableStream code paths. The warning explains the cause and tells the user to normalize their stream to a consistent chunk type.

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