supabase / supabase-flutter / 36606520173

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fix(functions): decode response bodies that are not valid UTF-8 instead of throwing (#1892)

## What

`FunctionsClient.invoke` decodes text bodies with
`utf8.decode(bodyBytes, allowMalformed: !isSuccessStatus)` at both call
sites in `functions_client.dart`, so an error body that is not valid
UTF-8 decodes with U+FFFD in place of the bad bytes and still reaches
the caller as the exception `details`. A success body stays strict, the
same way the JSON branch already rethrows on a malformed success body.

## Why

Both `utf8.decode` calls used the default `allowMalformed: false`. A
response body that is not valid UTF-8, for example `Café` sent as
ISO-8859-1 (`43 61 66 E9`) by an Edge Function or a proxy in front of
it, threw a bare `FormatException` out of `invoke`:

| Response | `main` | this PR |
|---|---|---|
| 200 `text/plain`, body `43 61 66 E9` | `FormatException: Unfinished
UTF-8 octet sequence (at offset 4)` | `FunctionResponse(data: 'Caf�',
statusCode: 200)` |
| 500 `application/json`, body `<html><body>Caf\xE9</body></html>` |
`FormatException: Missing extension byte (at offset 16)` |
`FunctionsApiException(statusCode: 500, details:
'<html><body>Caf�</body></html>')` |

The second row is the bug: a gateway error page in Latin-1 already fails
the JSON decode, and the fallback decode in the `on FormatException`
handler threw a second `FormatException` from inside the handler, so the
status code and body were lost and `on FunctionException` handlers never
ran.

functions-js reads these bodies with `response.text()`, whose decoder is
non-fatal and substitutes U+FFFD
(`packages/core/functions-js/src/FunctionsClient.ts`); this PR takes
that only for the error path. Valid UTF-8 decodes exactly as before.

## Not a breaking change

No public API changes. Bodies that decoded before decode to the same
string, and a malformed success body throws as before.

## Tests

`packages/supabase_functions/test/functions_dart_test.dart`, two new
cases driven by two ne... (continued)

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