supabase / supabase-flutter / 40907008793

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feat(storage): add byte-capped collect and atomic write-to-file for streamed downloads (#2103)

Adds two helpers for consuming a streamed storage download, following
the `storage.file_buckets.download_as_stream` spec. Closes SDK-2590.

## Changes

- `StorageByteStream` extension on `Stream<Uint8List>`:
- `collectBytes({int? maxBytes})` counts bytes as they arrive, after
decoding by the HTTP stack. Once the cap is exceeded it cancels the
subscription (closing the connection), discards what was collected and
throws `StorageDownloadTooLargeException`. It never looks at
`Content-Length`. Without a cap it collects the whole body.
- `writeToFile(String path)` writes to a new temporary file next to the
destination and renames it over the destination once the stream ends
cleanly. On any failure, including an abort, the temporary file is
deleted and the destination is untouched. The directory must exist. On
the web it throws `UnsupportedError`, through the existing `dart:io`
conditional import.
- `StorageDownloadTooLargeException` extends `StorageException` and
carries `maxBytes`.
- The helpers are extensions on the stream, so they work with whatever
shape `downloadStream` ends up with in SDK-2576. Nothing about
`downloadStream` changes.
- `sdk-compliance.yaml` registers the new symbols.
`storage.file_buckets.download_as_stream` stays `partially_implemented`,
since the remaining `downloadStream` drifts are still open in SDK-2576.

## Tests

Exceeding the cap (including that the stream is cancelled), a body
exactly at the cap, a gzip body counted after decoding through a real
local server, a failure halfway through the file write, an aborted
download, replacing an existing file, and a missing directory.

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## Summary by CodeRabbit

* **New Features**
* Downloaded byte streams can be collected into memory, with an optional
size limit. Exceeding the limit... (continued)

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