Make HttpApi injectable + real MockHttpClient tests

[Chemaclass]

Context

HttpApi constructs its client inline, so the HTTP layer can't be exercised with a fake transport in tests. The graceful failure path (transport/HTTP error → null → Backend "LnBits" unreachable) is only covered indirectly.

• src/Invoice/Infrastructure/Http/HttpApi.php — HttpClient::create() is called inside postRequestInvoice(); catches Symfony\Contracts\HttpClient\Exception\ExceptionInterface | JsonException → returns null.
• src/Invoice/InvoiceDependencyProvider.php — provides new HttpApi().

Goal

Make HttpApi injectable so its real behavior (success + transport-error → null) can be unit-tested with Symfony's MockHttpClient, with no network.

Scope / Tasks

• Add an optional constructor param HttpClientInterface $client = null, defaulting to HttpClient::create() (backwards compatible — new HttpApi() still works, and InvoiceDependencyProvider needs no change).
• Use the injected client in postRequestInvoice().
• Add unit tests using Symfony\Component\HttpClient\MockHttpClient:
  • 200 with a JSON body → returns the decoded array.
  • Transport error (MockHttpClient throwing) → returns null.
  • Non-2xx / malformed body → returns null (exercises the ExceptionInterface | JsonException catch).

Implementation notes

• symfony/http-client is already a dependency; MockHttpClient ships with it (no new dep).
• Keep the default-construction path so nothing else needs wiring.

Acceptance criteria

• HttpApi is constructable with a custom client; default construction unchanged.
• Tests cover success, transport-error→null, and bad-response→null with no real network.
• composer test-all green.