Typed response DTOs for LNURL pay-params & invoice

[Chemaclass]

Context

The two LNURL responses are built as loosely-typed associative arrays, documented only via array-shape docblocks:

• src/Invoice/Application/CallbackUrl.php — returns the pay-params array.
• src/Invoice/Application/InvoiceGenerator.php — mapResponseAsArray() returns the invoice array; generateInvoice() also returns the {status, reason} error array.
• src/Invoice/InvoiceFacade.php — array-shape docblocks on both methods.

Goal

Introduce typed, immutable response value objects (implementing JsonSerializable) for the pay-params and invoice responses, so construction is type-checked and adding protocol fields (comment/successAction/nostr) is safe — while serializing to the exact same JSON.

Scope / Tasks

• Add readonly DTOs, e.g. PayRequestResponse and InvoiceResponse (+ an error representation), implementing JsonSerializable to emit the current keys/shape byte-for-byte (pr, status, memo, successAction, routes, disposable, error; and callback, maxSendable, minSendable, metadata, tag, commentAllowed).
• Build responses via the DTOs in CallbackUrl / InvoiceGenerator. JsonResponse in the controller already JSON-encodes; passing a JsonSerializable must produce identical output.
• Replace the array-shape docblocks on the facade/generator with the DTO return types (or keep facade returning arrays via jsonSerialize() if that's cleaner for consumers — decide and document).
• Update tests to assert against the serialized shape (unchanged JSON).

Implementation notes

• The serialized JSON MUST stay identical (this is a wire contract used by wallets) — assert equality against the current expected arrays.
• Keep DTOs in src/Shared/Transfer/ or an Application/Response/ namespace consistent with the module layout.
• This is a refactor: no behavior/wire change, purely internal typing + DX.

Acceptance criteria

• Responses are constructed from typed DTOs; JSON output is byte-identical to today.
• Facade consumers get precise types (DTO or a typed array shape).
• composer test-all green.

Out of scope

• Adding new fields (covered by the LUD/Nostr issues) — this only re-types the existing shape.