Fix StreamHasher hashing bytes rejected by the wrapped writer

[Shubham-Padkonde]

StreamHasher.write currently hashes the full input before calling the wrapped writer. A short write therefore includes bytes that were never accepted, and a failed write changes the hash despite writing nothing.

Hash only the accepted prefix after write returns, preserving its return value. Zero-byte and non-blocking writes that return None leave the hash unchanged.

Added regressions for short, zero-byte, non-blocking, and failed writes. All four fail before the fix. All 104 unit tests pass on Python 3.13, and changed-file Ruff lint and formatting checks pass. No live Dropbox API calls were needed.

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