supabase / supabase-flutter / 28444101116

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fix(functions): drain error responses with a streaming content type (#1480)

## What

When an edge function returns an error status with a `text/event-stream`
content type, the SDK now drains and decodes the body into
`FunctionException.details` instead of handing back an unconsumed
stream.

## Why

The streaming branch assigned the raw stream regardless of status:

```dart
} else if (responseType == 'text/event-stream') {
  data = response.stream;
}
...
throw FunctionException(status: response.statusCode, details: data, ...);
```

On a non-2xx response that meant:
- `details` was the raw `response.stream`, so callers saw `Instance of
'ByteStream'` instead of the actual error body.
- the stream was never consumed, so the underlying connection was left
open (the auto-created `http.Client` can't be reclaimed).

Every other content-type branch already drains via `toBytes()`; only the
event-stream branch returned a live stream. The fix gates that on a
successful status:

```dart
} else if (responseType == 'text/event-stream' &&
    200 <= response.statusCode &&
    response.statusCode < 300) {
  data = response.stream;
} else {
  // drain + decode
}
```

## Not a breaking change

A successful streaming response still returns the live `response.stream`
exactly as before. Only the error path changes — from an unusable stream
+ leaked connection to a decoded body string.

## Tests

Added an `error-sse` route to the test client (status 500,
`text/event-stream`) and a test asserting `invoke()` throws a
`FunctionException` whose `details` is the decoded body (`'error:
boom'`). Without the fix `details` is `Instance of 'ByteStream'` and the
test fails.

---------

Co-authored-by: Lukas Klingsbo <lukas.klingsbo@gmail.com>

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