make VideoFromFile.get_stream_source() re-entrant and BytesIO-compatible

[bigcat88]

This PR fixes a concurrency issue in ComfyUI video inputs when the same in-memory video is consumed by multiple parallel async nodes.
Previously VideoFromFile could expose a shared BytesIO cursor, causing one consumer to advance the stream and the other to read truncated data or hit EOF.

The solution introduces _ReentrantBytesIO: a read-only, seekable BytesIO subclass that shares immutable bytes while providing an independent cursor per consumer. VideoFromFile.get_stream_source() now returns a fresh _ReentrantBytesIO instance for in-memory sources, preserving compatibility with downstream code that checks isinstance(x, BytesIO)

Other options considered:

1. Return a new BytesIO on every call (copy each time)

   Too expensive for large videos and parallel graphs.

2. Lock/serialize access to a shared BytesIO

   Avoids copying but removes parallelism (and still relies on cursor rewinds)

3. Write to a temporary file and return a path

   Compatible and re-entrant via independent file handles. The smallest implementation in size, can be found here

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Currently chosen: return a lightweight BytesIO-compatible re-entrant view (shared bytes, independent cursor)

Minimal overhead, preserves parallelism, and keeps compatibility with node packs that expect a BytesIO.