refactor: share the HTTP request pieces between the fetch layers

[spydon]

Tier 3 of #1572, last of four PRs. Stacked on #1646 (→ #1645 → #1644).

The tier 3 note said the fetch wrappers are shareable in pieces, not as one wrapper, since each is bound to its own exception type. These are the pieces, now in supabase_common:

Helper	Replaces
sendRequest(request, {httpClient})	httpClient != null ? httpClient.send(request) : request.send() in functions, storage (3 sites) and the Iceberg catalog
headerValue(headers, name)	case-insensitive header lookups
setDefaultContentType(headers, value)	storage's and functions' own "set Content-Type unless the caller already did" checks
responseMediaType(headers)	functions' (headers['Content-Type'] ?? headers['content-type'] ?? 'text/plain').split(';')[0].trim().toLowerCase() and the Iceberg catalog's contentType.contains('application/json')
tryDecodeJsonObject(body)	the "decode the error body, fall back to the raw body" dance in storage and postgrest

supabase_common gains a dependency on http (^1.6.0, the constraint every other package already uses).

Two bugs this surfaces

Each has a regression test that fails without the fix.

Postgrest swallowed typed errors for maybeSingle(). The decode and the maybeSingle handling shared one try/catch (_):

try {
  final errorJson = jsonDecode(response.body) as Map<String, dynamic>;
  error = PostgrestException.fromJson(...);
  if (_maybeSingle) {
    return _handleMaybeSingleError(response, error); // rethrows `error` for a real failure
  }
} catch (_) {
  error = PostgrestException(message: response.body, ...); // ...and catches it here
}

_handleMaybeSingleError returns null data for the "0 rows" case and rethrows otherwise, but that rethrow landed in the catch (_) and was replaced by a generic exception built from the raw body. So a maybeSingle() request that failed for a real reason lost its errorCode, details and hint. Splitting the decode from the handling fixes it.

A JSON error body with unexpected field types crashed. Removing that catch-all exposed the next layer of the same problem: PostgrestException.fromJson cast code and hint to String and required a String message, so {"code": 502, "message": "Bad gateway"} — a shape gateways really do return — threw a TypeError out of the builder. Both fromJson factories now read every field defensively, so any JSON object yields an exception. StorageException.fromJson had the same casts and the same exposure.

The matching storage bug, where a JSON error body that is not an object escaped as a TypeError, is fixed in #1644; here its inline guard collapses into tryDecodeJsonObject.

Not converted

Postgrest's own send site keeps managing its Client explicitly. It has to close a client it created even when the response body fails midway, and BaseRequest.send only closes its internal client when the stream completes normally.

gotrue and postgrest also keep setting Content-Type unconditionally rather than adopting setDefaultContentType: both always JSON-encode the body, so honouring a caller-provided content type there would mislabel the request rather than respect it.

Testing

New http_test.dart in supabase_common covers each helper, plus the three regression tests described above. melos analyze, dcm analyze and melos format clean; gotrue, postgrest, storage_client, functions_client, realtime_client, supabase, supabase_common and supabase_flutter suites pass. Capability matrix symbol and drift checks pass.

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