Add minimum remaining-data checks for fixed-size binary reads

[FUTURE-SL]

This change adds pre-read validation for operations where the required byte count is known in advance.

What changed:

• added EnsureCanRead(int byteCount)
• used it in InternalRead(Span<byte>)
• used it in ReadExactly(Span<byte>)
• used it in ReadString() after reading the encoded byte length

Why:

• fail earlier on seekable streams when there is not enough remaining data
• avoid starting partial reads for fixed-size values and length-prefixed strings
• improve behavior for truncated or malformed inputs without introducing hard size limits

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[huoyaoyuan]

This is unexpected behaviour change and degrades performance. It's the stream's responsibility to reject oversized reads.

[adamsitnik]

Stream.Length will often perform a sys-call. This is a performance regression.
Stream.Length can throw an exception for seekable Streams. This is a breaking change.

The PR has no tests that show the benefits. Please open an issue first and describe the problem you are facing.