Verify & fix invoice description_hash == sha256(metadata) (LUD-06)

[Chemaclass]

Context

For LNURL-pay (LUD-06), the bolt11 invoice's description_hash must equal sha256(metadata) where metadata is the exact UTF-8 string returned in the pay params metadata field. Wallets that verify this will reject a mismatched invoice.

• src/Invoice/Domain/BackendInvoice/LnbitsBackendInvoice.php — sends to LNbits:

  'unhashed_description' => bin2hex($metadata),
  'description_hash'     => hash('sha256', $metadata),

• src/Invoice/Application/InvoiceGenerator.php — builds metadata via LnurlPayMetadata and passes it to requestInvoice().
• src/Shared/Value/LnurlPayMetadata.php — the metadata string builder.

Goal

Verify and, if needed, fix that the invoice returned to the wallet carries description_hash == sha256(metadata) using the same metadata string advertised in the pay params — so real wallets accept the invoice.

Scope / Tasks

• Confirm what LNbits does when both unhashed_description and description_hash are supplied — determine whether they can conflict and which one LNbits honors. Send only what's needed (likely just unhashed_description, letting LNbits hash it, OR just the correct description_hash), backed by the LNbits API docs.
• Confirm bin2hex($metadata) is the encoding LNbits expects for unhashed_description (hex of the raw metadata bytes) and that the resulting invoice's description_hash matches sha256(metadata).
• If a mismatch is possible, fix the payload so the advertised metadata and the invoice's description_hash always agree.
• Add a regression test asserting the payload sent to HttpApiInterface encodes the metadata such that description_hash == sha256(metadata) (stub the HTTP layer and assert on the request body).

Implementation notes

• The pay-params metadata and the invoice metadata must be byte-identical — both already flow from LnurlPayMetadata; keep it that way.
• Back any claim about LNbits behavior with the LNbits API docs or a real response sample (do not assume) — see repo convention on evidence-backed external-API claims.

Acceptance criteria

• Documented confirmation (docs/sample) of the correct LNbits payload for description-hash LNURL invoices.
• Test asserts the sent body yields description_hash == sha256(metadata).
• composer test-all green.

References

• LUD-06: https://github.com/lnurl/luds/blob/luds/06.md
• LNbits payments API docs.