feat(postgrest): support aborting requests

[spydon]

Summary

Adds request aborting to the PostgREST client, following up on the discussion in #1560 (comment): now that the repo is on http ^1.6.0, in-flight requests can actually be cancelled via the Abortable API, so the reasoning that removed cancellation-based features from #1560 no longer applies.

Stacked on top of #1560 (base branch worktree-SDK-1270). Review/merge that first.

New per-request API on PostgrestBuilder:

final abort = Completer<void>();
Timer(const Duration(seconds: 5), abort.complete);
try {
  final data = await supabase.from('users').select().abortCompleter(abort);
} on RequestAbortedException {
  // The request was aborted before it completed.
}

• PostgrestBuilder.abortCompleter(Completer<void> completer) — aborts the request when completer completes. Available on the query, filter and transform builders, and preserved across further chaining.
• When triggered, the request throws RequestAbortedException (re-exported from package:http).
• Aborting short-circuits the auto-retry loop: an aborted request is intentional and is never retried.
• The abortable path is only used when a completer is set, so all existing requests keep their exact current send behavior.

Design notes

• The API name follows the proposal in feat(postgrest): abort requests #1368 (abortCompleter). supabase-js uses abortSignal; since Dart has no AbortSignal, a Completer<void> is the idiomatic equivalent. Open to abortSignal(Future<void>) instead if preferred, which would also allow Future.delayed-based timeouts directly.
• Aborting depends on the underlying Client supporting it (the platform IOClient / BrowserClient in http 1.6 do).

Request timeout

database.configuration.request_timeout is still not_applicable in the matrix. It is now technically feasible to build a real per-request timeout on top of this (a Future.delayed as the abort trigger), so that entry could be revisited in a follow-up if we want a first-class timeout option.

Compliance matrix

• database.using_modifiers.request_cancellation → implemented

Tests

packages/postgrest/test/abort_test.dart: aborting a GET, aborting an insert (POST), no-retry-on-abort, normal completion when never triggered, and preservation across chaining.

Closes #1368

[spydon]

#1368 already deals with this, so I'm closing this one.